This is Conwy Castle, one of the best-known historic buildings in Conwy County. It's preserved as-is, but is largely ruined, which isn't uncommon for European castles generally. The interiors were often wood, and usually these were destroyed by fire, often more than once. Once castles became obsolete militarily, the incentive for rebuilding eventually was lost. However, rot was also a factor. In the case of Conwy, I haven't been able to find out if fire was a factor, but maintenance was stopped in the 17th Century. The castle was originally completed in 1289 for James of St. George. One odd addition is a railway tunnel built through the castle mound and integrated into the castle's design, presumably for aesthetic reasons, in 1848.