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Your options are sort of limited here. You're asking: "In an age where people have not built any large above-ground structures, what sort of large above-ground structures are there?"

You need to either reach out to fantasy or think outside the box.

Natural, mazelike terrain

• Open-air passageways through an icy tundra, or cracks in its ice. Effectively a cave minus the ceiling.
• Mountains with winding goat-paths, and a spattering of caves.
• A spirit-touched ravine with winding paths between its walls.

Natural obstacles to make crossing land hard

• A land teeming with rivers.
• Great islands separated by vast water. Some may be connected by natural bridges (sandbars, stone arches connecting two cliffs), others by artificial bridges (sturdy bridges, vine/rope bridges, ziplines). Alternately they may be connected by underground passageways or nothing at all, with reliance on boats or other transport to cross - if anyone even lives there to offer such services.
• Somewhere with lava flow - current with glowing rivers, or recent with hot stones. Mind you, Lava comes in a lot of forms and could be tricksy - that page alone has a couple of photos of lava sealed beneath rock, so that it might just look like rock (or mysteriously flowing rock if it's not still) - until you step in it.
• Heavily overgrown forestry.

This section isn't comprehensive - other answers have provided locations that could fit here and I'm not sure I should pilfer them for the sake of making this list comprehensive.

Dwellings created by others who can build big things

• A raft city built on a lake. In the ancient Celtic age, this may have been quite a technological feat and wonder, and might be a natural expansion upon Crannogs. The Wiki article will fill you in on how common they were, but they're a fairly ancient concepts (though common only in Scotland and Ireland). Come to think of it, I wonder how it's anchored - it would be a shame if anything were to happen to those anchors...
• An ancient, crumbled city - ancient man, perhaps. Dilapidated castles, caved-in houses, no dungeons and a lot of missing rooves giving you open air.
• A faerie dwelling. Celtic mythology was full of spirits, and if your setting's anything like theirs, yours might have its fair share of them too!
• A city at the top of giant trees. Pathways connect the trees, as many or few as you want. Instead of large stone walls separating you from another point, there's just a large gap and a long fall - unless you have the resources to cross the gap, or climb down and back up again.
• The Lost City of Atlantis.

###Send them elsewhere entirely This is definitely cheating, but can let you have some fantastic structures that can't exist in the real world. Whatever those would be. Like the Maze of Tzeentch.

• There's a place where another plane meets this world and the players have to venture in.
• The heroes experience a spiritual journey in their sleep - ooOoOOoooOOOOooo!!

Your options are sort of limited here. You're asking: "In an age where people have not built any large above-ground structures, what sort of large above-ground structures are there?"

You need to either reach out to fantasy or think outside the box.

Natural, mazelike terrain

• Open-air passageways through an icy tundra, or cracks in its ice. Effectively a cave minus the ceiling.
• Mountains with winding goat-paths, and a spattering of caves.
• A spirit-touched ravine with winding paths between its walls.

Natural obstacles to make crossing land hard

• A land teeming with rivers.
• Great islands separated by vast water. Some may be connected by natural bridges (sandbars, stone arches connecting two cliffs), others by artificial bridges (sturdy bridges, vine/rope bridges, ziplines). Alternately they may be connected by underground passageways or nothing at all, with reliance on boats or other transport to cross - if anyone even lives there to offer such services.
• Somewhere with lava flow - current with glowing rivers, or recent with hot stones. Mind you, Lava comes in a lot of forms and could be tricksy - that page alone has a couple of photos of lava sealed beneath rock, so that it might just look like rock (or mysteriously flowing rock if it's not still) - until you step in it.
• Heavily overgrown forestry.

This section isn't comprehensive - other answers have provided locations that could fit here and I'm not sure I should pilfer them for the sake of making this list comprehensive.

Dwellings created by others who can build big things

• A raft city built on a lake. In the ancient Celtic age, this may have been quite a technological feat and wonder, and might be a natural expansion upon Crannogs. The Wiki article will fill you in on how common they were, but they're a fairly ancient concepts (though common only in Scotland and Ireland). Come to think of it, I wonder how it's anchored - it would be a shame if anything were to happen to those anchors...
• An ancient, crumbled city - ancient man, perhaps. Dilapidated castles, caved-in houses, no dungeons and a lot of missing rooves giving you open air.
• A faerie dwelling. Celtic mythology was full of spirits, and if your setting's anything like theirs, yours might have its fair share of them too!
• A city at the top of giant trees. Pathways connect the trees, as many or few as you want. Instead of large stone walls separating you from another point, there's just a large gap and a long fall - unless you have the resources to cross the gap, or climb down and back up again.
• The Lost City of Atlantis.

Send them elsewhere entirely

This is definitely cheating, but can let you have some fantastic structures that can't exist in the real world. Whatever those would be. Like the Maze of Tzeentch.

• There's a place where another plane meets this world and the players have to venture in.
• The heroes experience a spiritual journey in their sleep - ooOoOOoooOOOOooo!!

Your options are sort of limited here. You're asking: "In an age where people have not built any large above-ground structures, what sort of large above-ground structures are there?"

You need to either reach out to fantasy or think outside the box.

Natural, mazelike terrain

• Open-air passageways through an icy tundra, or cracks in its ice. Effectively a cave minus the ceiling.
• Mountains with winding goat-paths, and a spattering of caves.
• A spirit-touched ravine with winding paths between its walls.

Natural obstacles to make crossing land hard

• A land teeming with rivers.
• Great islands separated by vast water. Some may be connected by natural bridges (sandbars, stone arches connecting two cliffs), others by artificial bridges (sturdy bridges, vine/rope bridges, ziplines). Alternately they may be connected by underground passageways or nothing at all, with reliance on boats or other transport to cross - if anyone even lives there to offer such services.
• Somewhere with lava flow - current with glowing rivers, or recent with hot stones. Mind you, Lava comes in a lot of forms and could be tricksy - that page alone has a couple of photos of lava sealed beneath rock, so that it might just look like rock (or mysteriously flowing rock if it's not still) - until you step in it.
• Heavily overgrown forestry.

This section isn't comprehensive - other answers have provided locations that could fit here and I'm not sure I should pilfer them for the sake of making this list comprehensive.

Dwellings created by others who can build big things

• A raft city built on a lake. In the ancient Celtic age, this would have been as much of a technological feat and wonder as modern-day Dubai, as even artificial islands were extremely rare. Come to think of it, I wonder how it's anchored - it would be a shame if anything were to happen to those anchors...
• An ancient, crumbled city - ancient man, perhaps. Dilapidated castles, caved-in houses, no dungeons and a lot of missing rooves giving you open air.
• A faerie dwelling. Celtic mythology was full of spirits, and if your setting's anything like theirs, yours might have its fair share of them too!
• A city at the top of giant trees. Pathways connect the trees, as many or few as you want. Instead of large stone walls separating you from another point, there's just a large gap and a long fall - unless you have the resources to cross the gap, or climb down and back up again.
• The Lost City of Atlantis.

###Send them elsewhere entirely This is definitely cheating, but can let you have some fantastic structures that can't exist in the real world. Whatever those would be. Like the Maze of Tzeentch.

• There's a place where another plane meets this world and the players have to venture in.
• The heroes experience a spiritual journey in their sleep - ooOoOOoooOOOOooo!!

Your options are sort of limited here. You're asking: "In an age where people have not built any large above-ground structures, what sort of large above-ground structures are there?"

You need to either reach out to fantasy or think outside the box.

Natural, mazelike terrain

• Open-air passageways through an icy tundra, or cracks in its ice. Effectively a cave minus the ceiling.
• Mountains with winding goat-paths, and a spattering of caves.
• A spirit-touched ravine with winding paths between its walls.

Natural obstacles to make crossing land hard

• A land teeming with rivers.
• Great islands separated by vast water. Some may be connected by natural bridges (sandbars, stone arches connecting two cliffs), others by artificial bridges (sturdy bridges, vine/rope bridges, ziplines). Alternately they may be connected by underground passageways or nothing at all, with reliance on boats or other transport to cross - if anyone even lives there to offer such services.
• Somewhere with lava flow - current with glowing rivers, or recent with hot stones. Mind you, Lava comes in a lot of forms and could be tricksy - that page alone has a couple of photos of lava sealed beneath rock, so that it might just look like rock (or mysteriously flowing rock if it's not still) - until you step in it.
• Heavily overgrown forestry.

This section isn't comprehensive - other answers have provided locations that could fit here and I'm not sure I should pilfer them for the sake of making this list comprehensive.

Dwellings created by others who can build big things

• A raft city built on a lake. In the ancient Celtic age, this may have been quite a technological feat and wonder, and might be a natural expansion upon Crannogs. The Wiki article will fill you in on how common they were, but they're a fairly ancient concepts (though common only in Scotland and Ireland). Come to think of it, I wonder how it's anchored - it would be a shame if anything were to happen to those anchors...
• An ancient, crumbled city - ancient man, perhaps. Dilapidated castles, caved-in houses, no dungeons and a lot of missing rooves giving you open air.
• A faerie dwelling. Celtic mythology was full of spirits, and if your setting's anything like theirs, yours might have its fair share of them too!
• A city at the top of giant trees. Pathways connect the trees, as many or few as you want. Instead of large stone walls separating you from another point, there's just a large gap and a long fall - unless you have the resources to cross the gap, or climb down and back up again.
• The Lost City of Atlantis.

###Send them elsewhere entirely This is definitely cheating, but can let you have some fantastic structures that can't exist in the real world. Whatever those would be. Like the Maze of Tzeentch.

• There's a place where another plane meets this world and the players have to venture in.
• The heroes experience a spiritual journey in their sleep - ooOoOOoooOOOOooo!!

Your options are sort of limited here. You're asking: "In an age where people have not built any large above-ground structures, what sort of large above-ground structures are there?"

You need to either reach out to fantasy or think outside the box.

Natural, mazelike terrain

• Open-air passageways through an icy tundra.
• Mountains with winding goat-paths, and a spattering of caves.
• A spirit-touched ravine with winding paths between its walls.

Dwellings created by others who can build big things

• An ancient, crumbled city - ancient man, perhaps. Dilapidated castles, caved-in houses, no dungeons and a lot of missing rooves giving you open air.
• A faerie dwelling. Celtic mythology was full of spirits, and if your setting's anything like theirs, yours might have its fair share of them too!
• A city at the top of giant trees. Pathways connect the trees, as many or few as you want. Instead of large stone walls separating you from another point, there's just a large gap and a long fall - unless you have the resources to cross the gap, or climb down and back up again.
• The Lost City of Atlantis.

###Send them elsewhere entirely This is definitely cheating, but can let you have some fantastic structures that can't exist in the real world. Whatever those would be. Like the Maze of Tzeentch.

• There's a place where another plane meets this world and the players have to venture in.
• The heroes experience a spiritual journey in their sleep - ooOoOOoooOOOOooo!!

Your options are sort of limited here. You're asking: "In an age where people have not built any large above-ground structures, what sort of large above-ground structures are there?"

You need to either reach out to fantasy or think outside the box.

Natural, mazelike terrain

• Open-air passageways through an icy tundra, or cracks in its ice. Effectively a cave minus the ceiling.
• Mountains with winding goat-paths, and a spattering of caves.
• A spirit-touched ravine with winding paths between its walls.

Natural obstacles to make crossing land hard

• A land teeming with rivers.
• Great islands separated by vast water. Some may be connected by natural bridges (sandbars, stone arches connecting two cliffs), others by artificial bridges (sturdy bridges, vine/rope bridges, ziplines). Alternately they may be connected by underground passageways or nothing at all, with reliance on boats or other transport to cross - if anyone even lives there to offer such services.
• Somewhere with lava flow - current with glowing rivers, or recent with hot stones. Mind you, Lava comes in a lot of forms and could be tricksy - that page alone has a couple of photos of lava sealed beneath rock, so that it might just look like rock (or mysteriously flowing rock if it's not still) - until you step in it.
• Heavily overgrown forestry.

This section isn't comprehensive - other answers have provided locations that could fit here and I'm not sure I should pilfer them for the sake of making this list comprehensive.

Dwellings created by others who can build big things

• A raft city built on a lake. In the ancient Celtic age, this would have been as much of a technological feat and wonder as modern-day Dubai, as even artificial islands were extremely rare. Come to think of it, I wonder how it's anchored - it would be a shame if anything were to happen to those anchors...
• An ancient, crumbled city - ancient man, perhaps. Dilapidated castles, caved-in houses, no dungeons and a lot of missing rooves giving you open air.
• A faerie dwelling. Celtic mythology was full of spirits, and if your setting's anything like theirs, yours might have its fair share of them too!
• A city at the top of giant trees. Pathways connect the trees, as many or few as you want. Instead of large stone walls separating you from another point, there's just a large gap and a long fall - unless you have the resources to cross the gap, or climb down and back up again.
• The Lost City of Atlantis.

###Send them elsewhere entirely This is definitely cheating, but can let you have some fantastic structures that can't exist in the real world. Whatever those would be. Like the Maze of Tzeentch.

• There's a place where another plane meets this world and the players have to venture in.
• The heroes experience a spiritual journey in their sleep - ooOoOOoooOOOOooo!!