This castle has been ruinous for hundreds of years - as you can see, it was named as a "ruinous fortress" as early as the late 14th C. CADW have maintained the remains though, and the thickness of the walls speak for sturdy building in the first place.
Unusually, rather than the central keep found in most castles, the landward side of the castle (the back of it overlooks a cliff which drops to the River Teify, where coracles were used for fishing salmon in more recent times) has two immense towers for defence, and if you look at the entrance you will see that there are defences (not unlike those at Castell-y-bere).
Back later with more info, but I am being Summoned from Downstairs!
Oh dear - a day later! Here is some info. from my AA book: Castles in Wales:
Cilgerran is situated at the tidal limit of the River Teifi, so quite large ships would have been able to reach the castle, whatever the state of the tide.
In its early history, it is possible that Cilgerran was the castle from which Rhys ap Twdwr's daughter Nest was abducted by Owain, who was son of the Prince of Powys. This would have been early in the 12th C. A century later it was captured by William Marshall, Earl of Pembroke (1204).
The castle has a link with Castell y Bere, since Llewellyn the Great (who lived in the latter castle) captured Cilgerran in one of the Welsh raids he led. However, William Marshall's son recaptured it in 1223. He immediately set about rebuilding the defences to strengthen it by adding the two great round towers and curtain wall between them and beside them. He was also responsible for the rock-cut ditch in photo 3 below. Each of these towers had 4 storeys, with the windows facing into the inner ward. By 1275 the castle was derelict, the English incumbents having suffered a defeat in this region around 1258.
Edward III had the castle repaired in 1377, when there was the threat of French invasion. It was strong enough to hold out against Owain Glyndwr's uprising in the first decade of the 15th C.
HERE is a link to the CADW site information on Cilgerran.
HERE is a link to the Wikipedia entry.
A rather splendid Willow Knight.
The back of the castle, overlooking the river.