These include castles designed from the outset to have masonry defences and timber castles where the fortifications or significant building have been replaced in stone. This includes all the classic castle types such as Shell Keep, Great Tower and bailey, Enclosure, Concentric castles etc. These are the buildings which are what are generally thought of as castles and are well described in many books, web sites etc. Masonry is stonework bounded with mortar or, in a few cases bedded with clay. A few castles have dry-stone walls, these are listed under timber castles, since the dry-stone walling basically requires a similar level of expense and skill as earthwork defences.
N.B. Some buildings, although called 'castle', are not usually accepted as being 'true' castles and these may be listed elsewhere such as under fortified manor house or tower house.