This worked block of grey tufo represents (along with SU 5566) the southern threshold for the entrance to Room 6/16 from the central courtyard and slab floor 5410. It is worked in such a way as to accommodate a door (presumably wooden) with a locking mechanism. It is aligned on the same north-south axis with threshold 5491 to the north, which leads to the landing for the staircase by which the upper terrace is accessed. When both of these doors were open, a view of the staircase would seemingly be possible through Room 6/16. In terms of movement through this room, from south (5565/6) to north (5491), one would seemingly be presented with a number of important features: the uncertain feature within a gap in the slab floor (5661) just west of the central axis delimited by blocks 5674, 5675, 5676, 5677, and 5687; the wellhead within the slab floor (5661) in the northwestern corner; and the sunken dolium within a cut in the bedrock in the northeastern corner (5424). This must have been a fundamentally important space, as it represents the only means of accessing the staircase from the central courtyard and the main entrance(s) of the building, and a full reconstruction of its layout, uses, and appearance is, I suggest, key to understanding the purpose of the building.