The rectangle in the middle of the atrium floor is a shallow pool, called an impluvium, to catch water running in from the skylight overhead.

Traditional Roman mansions had a layout of foyer (fauces—”throat”) leading to the open-to-sky atrium, leading to the tablinum, a sort of office/reception room. There might also be little rooms (alae, “wings”) off the atrium or tablinum, and a nook or sleeping space for the doorman. These were the public areas of the house; anything beyond this (bedrooms, garden, dining room), though they might specifically be designed to be visible from the front of the house to show off paintings or mosaics or statuary, were considered private. Only family and good friends were allowed therein.

“Atrium” means “black place”—probably from its origin as the room where a family’s meals were cooked and the hearth for warmth was kept. There would be a smokehole above, and the walls would eventually turn black from the constant fire. By historical times, rich people had separate kitchens, and the poor generally either cooked over a little grate in a common area, or just went down the pub for sausage rolls.

The house’s altar (lararium) would usually be placed in the atrium, along with a lot of showpiece furniture. Mus’s house does not have a lararium.

Vitalis’ house is modelled on one that doesn’t have a pool in the center of the atrium, but instead has a floor sloping toward drains in the corners.