- No External Entrance
Too many secret wizard bases have secret entrances, or worse yet, not-so-secret entrances. That leaves way too much up for chance. Avoid building your base in a haunted house, at the bottom of a dilapidated well, or in the caldera of an old volcano - these places tend to crawl with monsters and adventurers. Instead, hollow out a void in the middle of solid rock somewhere and make sure teleportation is the only way to get there. Guaranteed to reduce your random encounter rate.
You're in a sealed subterranean wizard base. That makes for some seriously stale air. And air, mind you, can lead to all sorts of problems, like ruining potion ingredients, providing oxygen to lab fires, and giving nosy PCs lungfuls of nourishment. It shouldn't be too hard to (1) get a ring or something that gets rid of the need to breath and then (2) get rid of all of the oxygen in your base. Pure nitrogen, or better yet, some noble gas, is waaay better. Grab a ring of sustenance while you're at it.
- Magical Biometric Wards
Need to keep things secure? Most people use a password. Those people end up having to remember said password, and end up writing it down somewhere to be found. And guess who finds it? An adventurer, that's who. No, you're smarter than that. Use magical wards that simply check for your personal aura to activate. Presto. Forget the password.
- Alarm Spells Everywhere
Alarms are some of the easiest spells to cast. Deck the halls! Worried that all the noise will drive you insane? Don't be alarmed, there are often silent and mental versions available too.
- Mark your Stuff
So some nasty PCs have carried away your stuff. Make sure you've cast arcane mark on it (or the equivalent in your game system of choice). You should be able to scry it and locate it from wherever now.
- For Your Eyes Only
As a self-respecting magic user, everything you write is done with explosive runes. Except this text here. Because I'm such a nice guy.
- Dead Languages keep you Alive
You spend a lot of time working on your wizard tome. That thing has your entire life! The last thing you want is other wizards getting access to your precious face book. Learn a dead language that the PC's in your region would never bother to learn, and use that to make all of your annotations. Something like boggart.
- Methods for your Escape
Should something go south, you need a way out. Stash scrolls of teleportation around your base for easy access. Better yet, stash little tokens you can break in case of emergency to be automagically whisked away. Being gone is a whole lot better than being dead. Definitely make sure that your cages, cells, and obligatory oubliette have hidden ways for you to get out. And you'll probably want to ward them to only accept your exit, by the way.
- Everyone Loves a Fetch Quest
You know what every adventurer likes? Adventure hooks. If you run into a party (or rather, a party runs into you), open your trusty Monster Book of Monsters, flip to a random page, pick a random valuable body part, and tell the party: "I am so glad you could make it. I just happen to dreadfully need you to kill a _monster_
so that I can obtain its _body part_ to complete my spell to hold together the fabric of space-time." Refuse to answer any questions, and claim "I'm just an NPC" if need be. If they come back, give them something, congratulate them, then bid them leave. Head of Vecna
An oldie but goodie; this trap is set by placing an altar in your base with an old goblin head and some illusions to trick it up. Adding weak traps can help support its validity. Label your creation the "Head of Vecna" or something similar, and make sure that invaders would have stumbled upon descriptions of the Hand of Vecna and the Eye of Vecna, where you cut off your hand/eye and replace it with the magical item to gain some of the lich-king's powers. Only here, of course, the nosy PCs will intuit that they need to cut off their own heads to reap the reward. Requires hiring a cleaning crew every once in a while.
Additional Ideas (2)
As the saying goes, it's hard to find good help. But when you do, it can be even harder to keep it. The Shop Steward will help ensure your population of henchmen remain happy and focused on their task. No more henchmen caught sleeping, or gabbing over the firepit, or accepting bribes from those pesky so-called 'heroes' who are always trying to break into your sanctum.
The hidden entrance is a better kept secret when a more obvious, hard to achieve entrance is present. Having a tall, windowless tower with a broken staircase winding around its walls that lead to a door on top floor will look like the way in. Seeing that large chunks of the staircase are missing will lead adventurers to assume they must overcome this hurdle to reach the door at the top. When, in fact, the door at the top leads to a single, lonely room that looks to have been abandoned. The real entrance is a hidden door somewhere along the staircase.