unless you are using mods, the ground can only be raised and lowered so much, 10-12 squares

But yes I like doing this as well. I made a keep in my second world, and dig out below it and made a cool hidden dungeon. and there is a teleporter in it to take you to my scrooge mcduck inspired money bin. which rests on top of a really high mountain top, and has no entry doors.

...now if only they would make the gold piles stackable.