A hypocaust is a central heating system invented by Romans and used in houses of the rich.
It was a hot air system. The walls and floors of all rooms were made hollow so that hot air could circulate around them. The hot air was produced by burning wood in an outside room or shed, and letting the hot air rise into the wall spaces and under the floor.
This would have been smoky hot air, so the walls and floors had to be well sealed to stop the smoke seeping into the rooms. The walls were plastered, and plaster makes a very good seal.