Forbiddance

This spell is great for characters who have powerful enemies—and what character worth his salt doesn’t? Once cast upon an area, it prevents anyone with a different alignment from the caster’s from entering the area. It also seals the area against all planar travel into it, preventing people or monsters from making a surprise visit. Finally, it’s huge. You get a shapeable 60-foot cube per level—and each such cube is enough to cover fifty stronghold spaces.

The key here is to make sure that you find a caster with an alignment identical to your own. Otherwise, you’ll be shoved out of your own place.

Creatures with high spell resistance can bypass forbiddance. Furthermore, a successful Will save allows people to physically enter the area, so this is hardly foolproof security. An intruder could teleport right next to the area and then walk in, for example, so it’s up to you to take secondary measures as well. The first step is to make sure that the forbiddance spell actually covers the entire interior of a room. After that, you can figure out other ways to help seal up the room tight.

You can set up the spell so that anyone with the right password can bypass its effects. However, that increases the material component cost by another 5,000 gp per 60-foot cube.

Casting: 660 gp (Clr11) plus material components