A: well, for a newly designed dungeon, or a current laborotory, sure a few hired thugs to guard things.. but surely after a year or two of having to replace the dead, pay the others danger pay, unions, adventurers, you'd give in and hire a goblin sapping team to dig out a decent few holes, and a snirfnebblin (evil gnomes?) team to design a nice intricate trap that you can come and go from, but all others get horribly mashed up if they pass through.. (remember the CUBE? - slice and dice... OUCH) also, traps.. if done correctly, last a few hundred years.. so after your buried in your tomb (or as a necromancer you continue your lifes work) your safely secured behind said trap..

B: In the original D&D rules (you know, Basic, Expert, Companion & Master) magic users had to have a dungeon at the base of a tower. The idea was so that creatures would infest it, creatures that they needed to live in it to provide a source of components for potions and suchlike.

By doing this, they created a ready source of dungeons for adventurers to clear out. Also, the advantage of building a dungeon, that I realised in Baldurs Gate, is that you can loot the bodies of the adventurers who come to clean it out. Of course, the down side is that some may end up getting the better of you... (And of course, that the denziens that you have in your dungeon go on the rampage in the local countryside killing peasants etc.) I can not remember the mention of any unions though. But D&D was a US creation, and unions are viewed as commie scum by them.