At the Charles Street entrance is a groined and vaulted stone archway, with columns hewn out of single stones each 11 ft. high, giving access to a vestibule nearly 40 ft. square the most noteworthy features of which are four groups of clustered marble columns supporting a coffered ceiling with an octagonal groined vault terminating in a lantern.
The famous quadrangle in its loftiest part measures 62 ft. by about 113 ft. 6 in., and the spaces between the loggie are 60 ft. by 90 ft. It is covered in by a glass and iron roof of a very original design, and is in fact a skeleton panelled ceiling - the metal ribs and centres of each coffer alone being marked - above which rises a glazed iron roof, the whole forming one undivided structure of great strength. The ceilings of the loggie are of coloured tiles, the columns of polished granite, and below the principal course runs a frieze with coloured decorations.