Nice piece in Mother Earth News about the energy efficiency of underground Roman villas in Tunisia
"THREE BASIC FLOOR PLANS
Interestingly enough, most of Caesar's subjects who settled in Bulla Regia used one of three basic floor plans:  a vestibule running along three contiguous main rooms whose rear windows opened — in common — onto a large, deep air shaft,  a central courtyard, or peristyle, surrounded by rooms that had aboveground windows and openings in their ceilings, and  a hallway with shallow rooms on either side that incorporated windows set high in the upper walls. (The basement dwellings, though often smaller than those on the surface, frequently duplicated portions of the structures' ground-level floor plans.)
PLAN NUMBER ONE
The "House of Amphitrite" is a large version of the first design. (In addition to the three main rooms, it has two tiny chambers situated across the vestibule.) This residence, the floor of which is almost 17 feet below ground level, still retains some stucco wall panels and mosaics.
The inlaid lobby floor — picturing a female face framed by a leafy border — is in a state of excellent preservation, and visitors can still see where a marble fountain was once embedded in a nearby wall. The large central dining room has two columns facing the entryway, a vaulted ceiling, and a pastel floor mosaic portraying the "Triumph of the Marine Venus". (Years ago, an erroneous identification of the pictured goddess as Amphitrite, wife of Neptune, gave the house its name.)
PLAN NUMBER TWO
A dwelling known as the "Hunting Palace" is typical of Plan Two. Below the spacious courtyard (almost 27 by 32 feet) on the main floor, eight 16-foot-high Corinthian columns support the ground-level ceiling ... which is pierced with hexagonal openings to serve as air and light sources for the underground peristyle.
The rooms open to the north and west. Three bays, which are separated by two large columns, give access to a large dining room, whose decorated floor covers a circular storage cistern more than four feet in diameter. Baked earthen pipes in the ridged molding of the arched ceiling empty into the reservoir.
Simple mosaics and raised platforms indicate the one-time placement of beds in three sleeping chambers, which are copies of those upstairs. (Most of the rooms are rectangular, with two ceiling holes apiece to admit light and air.)
PLAN NUMBER THREE
One good example of the third floor plan is the "House of the Peacock". Fourteen steps descend about ten feet . . . to a passageway with rooms on both sides. The first chamber to the left is a sleeping room: It was identified as such on the basis of the bed platform against the back wall. The large room to the right opens — on the south side — to a smaller chamber which has an apse with an almost effaced peacock mosaic.
(Across the road from "the Peacock" is another interesting Plan-Three house. Its ingenious owner simply transformed two pre-existing deep cisterns into rooms when the family decided to "move down".)"
More on Bulla Regia (Great Photos!!!)