Well I'm going to assume that these guys are evil, so here goes:
15th level Cleric
1. Forbiddance - absolutely mandatory to protect your stronghold. The easiest method is to key it to a password. Otherwise, foes need to make a Will save or be damaged (unless they share the same alignment as you -- unlikely in this case). Also prevents things like teleporting and summoning into the stronghold.
2. Desecrate - can be applied to the entire stronghold. Surrounds it with a magic circle against good effect, bolsters undead significantly, and you can tie a spell to it. I like to place an invisibility purge that only hinders creatures of the opposite alignment or enemy faith.
3. Symbols (various) - these are quite powerful if placed on the floor. Fill the guardians of the room with constructs or undead, which are immune to all symbol types and have them repeatedly trigger the symbols each round.
4. Glyph of Warding (greater) - sprinkle these liberally around your stronghold, particularly near senstive areas. Pretty soon your PCs will become paranoid and (if they have a rogue) check for traps everywhere! It is best not to put these glyphs in obvious places. Put them instead in the middle of a hallway or a well-traversed chamber. You can give your own minions the password to bypass them.
15th level Wizard
1. Guards and Wards - the quintessential "I need to protect my stronghold" spell. However, can be very inconvenient for retainers. Use at your discretion.
2. Screen - this spell can be absurdly powerful. You can make your own personal chamber, a guard room, or whatever look like nothing is there (illusion). If the PCs don't make their Will saves or have true seeing up, then they are screwed. The enemies in the room can attack the PCs all they want with spells and ranged weapons and the PCs, still cannot disbelieve unless they save!
3. Mind Blank - prevents nosy PCs from divining the location of your evil Wizard. Even foils the vaunted discern location.
4. Antipathy - useful for casting on important stuff, like your spell book, sleeping chambers, research lab, etc. Of course you have to know the majority alignment or race of the PCs, but this shouldn't be too difficult.
5. Demand + Symbol (Death, Insanity) - if you know the PCs are in your stronghold, you can demand that one of them trigger a symbol you placed around your stronghold.
6. Contigency + Dimension Door - if and when the PCs get the better of you, this mental trigger will allow you to escape even if you are grappled or paralyzed.
