what are its notable architectural features?
Little is known about the design of the first castle and Gundulf's castle was enclosed by a stone wall.
Rochester Castle
Little is known about the design of the first castle at Rochester as it has since vanished, and even the exact site is uncertain. It most likely took the form of a motte-and-bailey castle, with a mound and an outer enclosure defended by a timber palisade and earthen banks. Boley Hill has been suggested as the site of the first castle, an outwork reinforcing the castle's defences, or an abandoned siege castle like those documented in the 1088 siege. Boley Hill is a natural outcrop of rock, and could have acted as a motte.

The castle built by Bishop Gundulf in the late 12th century was enclosed by a stone wall.
when was rochester castle built? UNANSWERABLE who built it? Gundulf, Bishop of Rochester which kings had the possession of the castle? King John and his son Henry
wiki:12875640
Rochester Castle [SEP] Architecture
The third floor had a second chapel and access to the roof, and may have held additional accommodation. The current entrance in the north-east occupies the approximate location of the main gatehouse constructed by Gundulf and then rebuilt during 1249–1250. It was pulled down in the 1870s when the enclosure was converted into a municipal garden. An engraving from 1735 by the Buck brothers gives some indication of the gatehouse's form: the gate was an archway between two towers projecting from the curtain wall. It was reached by a stone causeway across a ditch, rather than a drawbridge. A tower containing a postern gate was located in the north-west corner of the enclosure, built at the close of the 14th century to guard the bridge over the Medway.