Complex sentence: It is primarily a list, not of castles that were built in England, but of buildings and remains that have survived; in almost every case the buildings that survive are either ruined, or have been altered over the centuries.
Split into simpler sentences: It is primarily a list, not of castles that were built in England, but of buildings and remains that have survived. In almost every case the buildings that survive are either ruined, or have been altered over the centuries.