Most castles throughout Britain fell into disrepair within 40 or 50 years of being built.
If they weren't ''lived in'' by a noble family, then they were not maintained at all...because the conflict they were built for was long since over.
The lead was stripped from the roofs, which meant that the roofs fell in, and then the floors beneath them too.
All the gleaming white render fell off the external walls, and exposed the haphazard stonework we see today.
Imagine what they would have looked like when new?
The Civil War in the 17th Century meant that many castles were garrisoned again. But, at the end of that war Cromwell ordered them to be part demolished so they could not be used defensively again. I believe that the tower near the Railway line at Conwy Castle was brought down for that reason... and then re-built by the railway company.