TOWER OF LONDON: ST. JOHN'S CHAPEL,
On the second floor of the White Tower or Keep, of the Tower of London, that part which William the Conqueror built in 1078, is the Chapel of St. John, a singularly interesting specimen of Norman architecture - the best preserved, indeed, in all England. Nothing could be simpler, nothing more effective. Its massive pillars, rising from square bases and ending in cubical capitals, support a triforium gallery, and the intervals between these pillars are spanned, like the windows, by semicircular arches. The walls are of coarse masonry; the barrel vaulting of the roof and the groining of the aisles are rougher still. The fortress also contains the chapel of St. Petrus ad Vincula, shown on the next page.
source: The Queen's London : a Pictorial and Descriptive Record of the Streets, Buildings, Parks and Scenery of the Great Metropolis, 1896