Tombs!

From Introduction:

To adventurers, what lies within these ancient tombs holds more importance than their appearance or the reason for their creation. Unless a tomb has already been ransacked, everything within will be very, very old, perhaps older than any kingdom now in existence. The original makers of the tombs, ages ago, placed these objects within. The older a tomb is, the more likely that it contains both great riches and powerful guardians. For a tomb to survive intact for so long, the guardians must have been puissant enough to crush all comers.