The baron William d'Aubigné had quickly garrisoned the castle at Rochester with about 100 knights and 50 'men-at-arms' and some others from a local village, but apparently was short on provisions and other defensive means to hold out against a long siege.

The attacking King John is said to have amassed a 'considerable force', it seems overwhelmingly filled with Flemish and Poitevin mercenaries (who are quoted as participating in various discussions); apparently John came with large enough long-term siege resources that they took over the local cathedral and turned it into provisional stables.

The story of the assault is this: after cutting off the main road to London, King John's started a siege with petraries - a fairly generic latin term for a rock throwing engine. Those didn't penetrate the curtain wall and so the attackers started mining it. Once the curtain wall was penetrated the siege moved to the inner keep to which d'Aubigné and the defenders had retreated. This was in turn mined by John's forces on one corner and that part of the keep fell outwards:

A letter survives in which John asked to be sent 40 of the “fattest pigs”, whose fat was used to fire the timbers in the mine. The defenders continued to resist from behind the cross wall that divided the great tower in two, but their cause was now hopeless and they surrendered on 30 November. (Purton, History of the Early Medieval Siege 420-1220, Boydell, 2009)

According to chroniclers, a baronial relief force came up from London to aid d'Aubigné however they were cut off from approach by the aforementioned king's blockade; these troops returned to London to 'drink and dice' and 'other things besides'.

The main sources outside of some letters about the event are Roger of Wendover's Flores Historiarum and Walter of Coventry's Memoriale fratris Walteri de Coventria . Wendover in particular is more hostile to the monarchy.

Aside from Purton's book, see:

• Bradbury, Jim. The Medieval Siege (Boydell & Brewer, 1992)