The rectangular keep was the first part of the castle to be built, probably in about 1210 by Llywelyn ap Iorwerth. Amusingly, local tradition claims the keep was also his birthplace, in 1173. That's quite a trick. It's somewhat more likely that an earlier fort existed in the valley.
It may seem to be remarkably well-preserved for an 800-year-old structure, but the upper levels were extensively restored in 1848-50 by Lord Willoughby de Eresby, with a degree of reinterpretation – the crenellated battlements are fanciful.

The 'extension' in the south-western wall housed the keep's latrine, whilst the corner at the lower right of the image, outside the keep, accommodated a bread oven.