It appears to me that many castles in Britain and Ireland are ruined and appear to have not been maintained in hundreds of years. The classic examples in my mind are Tintagel Castle in England, Dunnottar Castle in Scotland, though there are many others. What would the reasons be for so many seaside castles to fall into ruin? Is it mostly the elements? Remoteness? Ceased being useful for defense? Slighting? This could apply to other areas as well besides Britain and Ireland, so don't feel limited in response.
Well, while I can't speak much on coastal castles within the UK, I do know a little bit about castles closer inland on the Republic of Ireland.
There are over 30,000 medieval castle and castle ruins within the Republic of Ireland. There are so many that if you grow up in Ireland, you're probably a stone throws away from some sort of castle or what used to be one. (Personally, I grew up within walking distance of two)
And then it becomes a simple reason of financials. If you inherit land that happens to contain castle/castle ruins, or buy land that happens to contain some - Maintaining that castle might be a massive undertaking. People might buy that land simply because it's good for farming, local trade etc..
It simply just comes down to how many castles there are and the price of keeping something like that maintained. Taking care of a castle just simply isn't a profitable undertaking unless it's an historically important castle.
[http://i.imgur.com/OEhBnde.jpg](Here is map containing the locations of every significant castle in Ireland)
It becomes pretty evident after looking at it that there's just simply not going to be enough resources to maintain all of them.
Again, I don't know why castles wouldn't be maintained in the UK, but I would assume it's for pretty much the same reason.
