The Mirage Arcane spell description says the illusion has tactile elements. What exactly does this mean? If I create a structure, will it be 'tactile' enough to stand on the second floor of? If I make a wall in the area, will it stop creatures from passing through?
The spell does exactly what it says:
you can alter the appearance of structures, or add them where none are present.
Creatures with truesight can see through the illusion to the terrain’s true form; however, all other elements of the illusion remain, so while the creature is aware of the illusion’s presence, the creature can still physically interact with the illusion. (PHB5e p.260)
So, as written, you could make a 50 storey building and stand on the roof even if you can see it isn't real!
Similarly, you could lay down a 300 foot high wall.
What does can mean?
In the context "can" means the creature is able to still physically interact with the illusion. When taken with the fact the creatures without truesight (including the caster) must physically interact it clearly gives such a creature the option to treat it as physically there or not at its discretion. Whether this is a one off election or it can decide from turn to turn is a DM call.
There's ambiguity, but it should be pretty real. It explicitly allows you to turn terrain impassable and to change whether terrain is difficult or not.
So you can create a series of rock formations that is impassible, and the only change listed for knowing it's an illusion is that it turns see-through.
It's not spelled out as explicitly for buildings, but it says you can make structures too, and doesn't have any instructions limiting the effect from the abilities it does spell out.