History Ruins

From what I understand, this was originally a royal residence of Ada, Countess of Holland. The walls make sense, as she was later deposed by her paternal uncle.

At first I was kind of skeptical about this smallish structure truly being a "castle," but:

A castle is a type of fortified structure built during the Middle Ages predominantly by the nobility or royalty and by military orders. Scholars usually consider a castle to be the private fortified residence of a lord or noble. This is distinct from a mansion, palace, and villa, whose main purpose was exclusively for pleasance and are not primarily fortresses but may be fortified.

Use of the term has varied over time and, sometimes, has also been applied to structures such as hill forts and 19th- and 20th-century homes built to resemble castles. Over the Middle Ages, when genuine castles were built, they took on a great many forms with many different features, although some, such as curtain walls, arrowslits, and portcullises, were commonplace. --WP

So I guess there are 'castles' and 'genuine castles,' FWIW.