Frequently the great Wall aligns itself with the northern wall of the forts, as at Borcovicium, Æsica and Amboglanna; but sometimes, as here at Cilurnum, it strikes the fort about one-third way along the eastern and western walls, leaving one-third, including a gateway, to project northwards into the enemy's country. This looks as if the garrison could not have lived in a constant state of warfare.
Fig. 11.—Gate of a Fort on a mosaic in the Avignon Museum.
From Collectanea Antiqua. (After John Ward.)
Streets run between the north and south gateways and the east and west, in every case. Where they cross are the central buildings, which were called the "Forum" by Mr. Clayton. But that is a civil term. Probably the more correct term is "Principia," to indicate the H.Q. building of a military unit.
This always includes an open courtyard, surrounded by a covered colonnade, the bases of whose piers are still to be seen here at Chesters, as also the gutter-stones to carry off the drippings from the roof.
Roofing-tiles are found, made of a grey shaly sandstone, which would readily cut into thin slabs. They are almost square, and were hung angularly, from one corner, on nails driven into wooden beams, as the rust in the holes still shows. This kind of roofing slab is used up to the present day, but it is hung on a wooden peg. At one period they used to be hung on sheep-bones thrust in under the beams.
The Romans also used red pantiles, specimens of which are found here.
The entrance to the Principia always faced the main gate of the fort (the north gate at Cilurnum, the east gate at Borcovicium). At the far end was a series of five small office-rooms, of which the middle one was the most important. It was the sacellum or Chapel of the Standards, the centre of the religious life and of the esprit de corps of the cohort or ala which occupied the fort. It was a constant feature of all Roman forts. The standards themselves, which were deposited there, were objects of worship. They bore a medallion or effigy of the reigning Emperor, and thus these Chapels were the official centres of "Emperor-worship," a cult actively propagated by the government for political motives. The fact that Christians refused to join in this cult, and thereby committed a political offence, was the initial cause of their persecution.