Sweating Room

View of the unique circular structure in the sweat room (laconicum) of the bath in Herod the Great's Third Palace at Jericho.

Evidently the floor, now missing was supported by beams stretched from the circular outer wall to the central circular structure.  "in all likelihood, it was heated by means of burning coal placed on special trays (braziers).  Rooms with a similar plan were common in Italian bathhouses of that period, some of the serving as (frigidaria) [cold rooms] and others as (laconica) [sweating rooms] (Netzer, 2008 p. 65, 391).

In some sources this room is labeled as a  frigidarium.

The walls of the outer ring of this building are made of opus reticulatum and opus quadratum. 

The stones that are laid diagonally are called opus reticulatum.  The long portion of each brick, wedge–shaped, but not really visible, is set in plaster that faced the rubble core.  The opus reticulatum was then covered with layers of plaster and/or frescos—see the bottom of the wall for plaster remnants.

The corners of the wall, where the wall transitions into two semi–circular niches, are made out of opus quadratum.  These stones are actually rectangular in shape and are laid like bricks are today.

These types of walls were very common in other parts of the Roman Empire for they could be mass–produced inexpensively—typically mud bricks would be formed and "fired" by unskilled labor.  BUT here, Herod actually used carved pieces of STONE, not mud bricks!

This type of wall is not common in buildings constructed during the days of Herod the Great.  Netzer believes that after the visit of Marcus Agrippa , Augustus' son–in–law and confidant, in 15 BCE, that Agrippa sent workmen to assist Herod in his remaining building projects (Herod died in 4 BCE) which included the building of this, the "Third Palace of Herod" at Jericho.

See Netzer, Ehud, and Rachel Laureys–Chachy. The Architecture of Herod, the Great Builder. Grand Rapids: Baker Academic, 2008, pp. 314–15.