(goes "Hear, hear," to Thorfinn's post, and adds the following
"I am a Dragon. I am a Red Dragon. I breathe Fire, and Fire heals me. So, naturally, I like to spit powerful Fireballs at anybody who dares challenge me. So, naturally, anybody who wants to challenge me should look for ways to gain Fire Resistance, to gain immunity to my Fireballs. I think I'll litter my dungeon with items that grant Fire Resistance, or are otherwise potentially lethal to Dragons in general and me in particular."
"I am a Vampire. I am Undead. I have many other Vampires and a few Ghasts working for me, all of whom are also Undead. Vampires can only be truly killed by pounding a Wooden Stake through their heart. I think I will place lots and lots of Wooden Stakes in various places in my dungeon, and also a weapon that instantly destroys any Undead creature with a single hit."