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 {90} "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. Here, in the _sacellum_, the deferred pay of the soldiers was deposited until it became due to them on their discharge. Here also were kept the army pay-sheets, an actual example of which has been found in the sands of Egypt. At Cilurnum a strong room has been constructed, evidently in a later and more disturbed period. It is entered by steps leading down from the _sacellum_, and lies under the office-room next to the _sacellum_ on the east, blocking that room in such a way as to prove it could be no part of the original plan. Early in the nineteenth century the heavy oak door of the strong room, studded with nails, was still in place, but it perished when exposed to the air. A slab of stone has been thrown (probably by invaders) across the flight of steps, so that in descending them one has to stoop very low. The arched roof of the strong room is formed by a series of large stones "stepping over" each other; and its back wall is also "stepped over," so as to make it incline towards the roof. On my last visit I saw a blackbird's nest neatly tucked in between the stones of this roof. It contained three young ones, with their mouths {91} perpetually open--creatures of one idea, for the time. There had been five eggs, but tourists had taken two away, as mementoes of Cilurnum.