According to Writer and Director Jonathan English, the bloody hackings of arms and legs were not done with CGI, but with old-fashioned prosthetics.
Loosely based on the life of William d'Aubigny, who commanded the Rochester garrison. Historians do not agree on the exact number of forces defending the Rochester Castle, with estimates ranging between ninety-five and one hundred forty knights supported by crossbowmen, Sergeants, and auxiliaries. When King John took the castle, most nobles were imprisoned or banished. The French forces did not arrive in England until six months after the end of the siege.
After the first attempt by King John's army to take the castle, King John (Paul Giamatti) can be seen eating a peach in his tent. When the real King John died in October 1216, his death was attributed to poisoned ale, poisoned plums, or a "surfeit of peaches".
According to Writer and Director Jonathan English, Daniel O'Meara really did eat a beetle during the starvation portion of the siege, but he's not sure the actor swallowed it.