During the English Civil War many castles and fortified houses were slighted by the Parliamentarians to stop them being used by the Royalists. Most of the destruction was in Wales, the Midlands, and Yorkshire e.g. Pontefract Castle. The coastal fortifications were spared by the Commonwealth, as they might have been useful for hindering a Royalist or foreign invasion.
I haven't been able to find how many castles and fortified houses were affected.