Why is Chepstow Castle so big?

Chepstow Castle is the oldest surviving post-Roman stone fortification in Britain. Situated in Monmouthshire in Wales on cliffs overlooking a bend in the tidal part of the River Wye, it was the southernmost of a chain of castles built in the Welsh Marches. Construction began in 1067 under the instruction of the Norman Lord William FitzOsbern. It was originally called Striguil, which means River Bend.