It must be said that the old walls could not have sustained a pounding on their own.. Donnington was a comparatively modest stronghold and certainly not designed to withstand powerful artillery. In preparation for the siege, Sir John Boys constructed a series of earthworks on the slopes around the castle. These, with their projecting bastions, are rare survivals of Civil War fortification.

The castle followed a quadrangular layout except that the rear bowed outwards in short, straight sections. There were round corner towers and two intermediate square towers on the longer sides. Owing to the siege or subsequent slighting only the footings of the curtain and its towers remain, but the handsome gatehouse has come down to us virtually intact, lacking only its roof and floors. The outer angles are clasped by boldly projecting, cylindrical towers which rise considerably higher than the main gatehouse.