This is a medieval fortress in Caernarfon, Gwynedd, north-west Wales cared for by the Welsh Government's historic environment service. There was a motte-and-bailey castle in the town of Caernarfon from the late 11th century until 1283 when King Edward I of England began replacing it with the current stone structure. The Edwardian town and castle acted as the administrative centre of north Wales and as a result the defences were built on a grand scale. There was a deliberate link with Caernarfon's Roman past and the Roman fort of Segontium is nearby.
Source: Wikipedia