Stepwells
In areas where water availability varies a lot through the year, you build reservoirs to hold it. And then build stairways down the side to make it easier to go get it in the dry season. And as long as you've got all those mason and laborers, why not add some decoration and cool architecture?
And some secret doors, only accessible in certain seasons?
Of course, after the local civilization collapses, who knows what unearthly horrors will move in.