One of the highlights of Ptolus is that the whole city sits on top of layer upon layer of catacombs. So, the house clearly needs a sub level. This level has almost no detail. The section on the bottom is meant to be the wine cellar, storage and so on. The circular rooms near the center are suppose to have a well and some magical water purification and circulation gizmos. The top right is meant to contain a number of magical workshops (for making scrolls and wands and such). The center rooms are an extension of the armory on the first floor, sort of a weapons depot/vault kind of thing. The bits on the left connect the towers and would contain shelters and maybe cells.

The stairs at the top left exit in the carriage house, behind a stack of barrels. Also, you can see some of the piping that feeds the various water features on the first floor, with waste pipes leading to the sewers.