Well, one way to do is to make the first cave level incredibly safe, but with only a few resources unique to that level, and little else. Once you access the second level(hopefully with a unique item from the first), there are more valuable resources, but it is much more dangerous. It would be progressively harder the more levels that were added, but the rewards would be progressively greater.
Actually, to make a stone wall, you need 2 Cut Stone, which costs six stones. You will always get one stone back from destroying a wall, and you get six stone walls for every two cut stones, I.E. as long as you don't reinforce walls, you will be able to place and replace walls. (Assuming the stone required for building a hammer is not an issue)