You can also build over tomb chambers, using them as your cellar. There are many disadvantages to using tomb chambers and troll caves -- if you store things in them, you just have to drop them on the floor, for instance, unless a troll cave already has containers -- but the huge advantage is that nothing can get in and attack you. Even in the very unlikely event that your entire structure outside the cave or above the tomb chambers gets demolished, you will still be safe. The yellow mushrooms in tomb chambers continually respawn as well (enemies and trolls don't), so you can harvest them for a low level food and to make stamina potions.