Dolbadarn Castle was built by the Welsh prince Llywelyn the Great during the early 13th century. Originally there were three towers, a hall and an East building within the walls on the hilltop, but only the round tower survives to any height. It was taken by the English in 1283. They removed timbers from it to build Caernarfon castle, but parts of the castle continued to be used as a manor house into the 14th century.
The keep is still an impressive structure, and the internal spiral staircase can still be accessed to climb from first to second floor levels, though the timber floors no longer exist. There are fine views from the castle mound.
Once you know where the castle is (it is visible from the narrow-gauge railway, near the LLanberis stop), it is easily reached on foot from LLanberis. There is no admission charge.