What printed spells and magic items would be especially useful for an 8th level Wizard to cast or craft to create a fortress?

Question

The scenario is as follows - an 8th level Wizard, Dormouse Bladefist, has a cave mouth he wants to defend at all costs. He has some money and magic materials, and can probably craft some weaker magic items. As a wizard, he has his spellbook, and can reasonably copy any other spells into it accessed through his Wizard's Guild that he currently lacks. He has access to labour from a local village, and the services of 7 Goblins, 3 Bugbears, and a Bulette who are relatively loyal to his cause.

Some old fortifications litter the cave mouth, but will not serve his purposes as they are old, broken, and mostly inside the cave.

He has time to prepare - several seasons, perhaps years, before enemies come to try to gain access to the cave mouth. He has no knowledge of what kind of enemies will seek entrance, but is determined to do his best to prepare for whatever he can.

Which printed spells or magic items would best assist him in this endeavour? I am specifically looking for long-lasting spells, ritual spells, magic items that would be especially useful in defending a static structure, what magical traps if any an 8th level Wizard could conceivably craft. Spells, items, traps, that would be especially efficacious in defense of a static location, rather than just powerful in general or otherwise, to give the impression of an intelligent man who has put some thought into what arcane defenses he can muster and been given the time to muster them.

Answer 1

Spell options

There are multiple options up to spell level 4 that are useful to fortify an area, if you have time to cast them.

• Spell Level 2: Arcane Lock (25 gp), if you embed any doors to provide access. Makes them harder to break down, and hard to pick.

• Magic Mouth (10 gp), a cheap alarm mechanism that never tires.

• Nystul’s Magical Aura, repeat 30 days to make permanent. Can mask the auras of magical objects to hide the presence of magic entirely. (I think it may be up to the DM if you can mask spell effects such as those of a Glyph of Warding, but consensus is yes). This also could mask illusions, but until you gain access to upcasting Major Image or Programmed Illusion on spell level six, there are no good permanent ones.

• Spell Level 3: Glyph of Warding (200 gp). This is your all-purpose power tool. You say the wizard has "some money", so shenanigans with stacking 100s of these along the cave entrance are probably off, but even a couple can be used to good effect with some thought. As you can put so many other spells in here, the possibilities are very varied.

• Spell Level 4: Private Sanctum. If he can repeat it for one year it becomes permanent, and it looks like this may be the case. If not, if he needs to block entrance via teleport et al., as the duration is 24 hours, he may still opt to just cast it every day. I think this is essentially a must-have if he expects any kind of magic-wielding opposition.

• Stone Shape, already described in detail in Pepijn's answer. Probably the strongest with the objective to fortify the cave mouth until you'll get access to Wall of Stone later on, also excellent camouflage.

• Fabricate, this depends a bit on access to raw materials, but again, with months of time he has a lot of slots here to create blocks of fortifications from stone and steel up to the size of a 5-foot side length cube. Rough Portcullises, Steel Vault Doors and so on. If it's Rock or Ore, material components are essentially free.

Lastly, you may be able to employ spell Level 1 Find Familiar (10 gp). Does not fortify in and of itself, but can act as a spy/alarm mechanism against approaching enemies, for example if you get a hawk to circle above and survey the area.

Non-magical options

In addition, with a whole team of Goblins and a Bulette and free peasant labor, and several months of time, I would not discount normal, mundane traps and fortifications: pit traps, cave-ins, building thick walls, crenellations, murder holes etc.

Also just posting some spies in the village with a carrier pigeon to alert him may be of use, if the village is a likely stopover for attackers.

Not everything needs to be a spell.

The game does not have any rules as written to detail how much these cost, but there are some guidelines on traps in the DMG on p. 120 and following.

Answer 2

The Stone Shape spell might be ideal.

With enough time to prepare you could fill the entrance with an assortment of appropriately sized stone boulders and shape them to block the entrance over multiple castings of the spell. Their might be seams but they would be air tight and you can shape the sections to lock into each other.

Advantages

• Considerably fortified, an entrance is only ever as strong as the surrounding material. If the entrance would be thick sheets of metal the enemy could just dig another entrance into the cave anyway.
• Decent camouflage that can't be magically detected like illusion spells.
• In case a barrier without active fortifications is considered too risky, you can still set up further fortifications right behind it.

Disadvantages

• Entering the cave yourself. You could include a stone door with working hinges, the spell specifically allows this, but that might ruin the camouflage. At the very least it would considerably reduce the size of the entrance which should make it easier to defend.
• Any enemy who knows for sure there used to be a cave entrance right there might not be fooled.
• If you are a DM and not a player, camouflage might not be the type of encounter you are aiming for.

All things considered I think this method is a decent first line of defense.