A county in North Wales, Gwynedd is known for its negligible population and substantial cultural cache; particularly the castles and fortresses that still stand today. Although there are numerous ancient castles in this region, a few of which are UNESCO-certified, there are three that are truly essential to an adventure experience.
Criccieth Castle makes the list less for its imposing structure or cultural heritage (it's a small castle ruin) than for its unmatched views over Cardigan Bay and the Snowdonia Mountains. Secondly, Harlech Castle remains standing as an example of King Edward's conquest of Wales, a large fortification that is perched on a hill overlooking the Irish Sea. Lastly, on the Isle of Langsley, Beaumaris Castle is a semi-ruinous, concentric fortification that has a moat and numerous inner and outer towers.
You could spend a whole day visiting these ancient attractions, whether your accommodations are in Wales or in English city of Manchester, which is a 2 to 3 hour drive to the east.
Note:Other amazing castles worth noting in Northern Wales are Dolbadarn Castle, Caerphilly Castle, Caernarfon Castle, and nearly a hundred others, making Wales the unofficial castle capital of the world. The map location and included picture is of Harlech Castle.