At the beginning of the 2nd century A.D., the Trajan Bath designed by Syrian architect Apollo Dolos determined the basic form of the royal bath: the main building was rectangular and completely symmetrical, with a hot water hall, a warm water hall and a cold water hall on the central axis; There are entrances on both sides, dressing rooms, massage rooms, rooms coated with olive oil and soap, and steaming rooms. The hall and rooms are arranged in a certain order of fitness and bathing; Boiler room, storage room and slave room are underground; There are pipes under the bathtub, in the wall and in the vault, which are used to pass hot air and smoke to keep warm.

The Karakala Bath, Diocletian Bath and Constantine Bath, which were built later, were all built in general imitation of this.