For those who haven't read it 'The Sea Kingdoms': Alistair Moffat, is an excellent introduction into how sea travel was a key to the Celtic Kingdoms.

http://www.amazon.com/gp/product/0002572168/002-3498906-4742427?v=glance&n=283155

As an aside, standing in the killing zone of Beaumaris Castle, Edward 1st's
(Boo! Hiss!) almost perfect defensive monument is one of the few places where I've actually felt an 'atmosphere'. A 'hair standing on the back of the neck' type of atmosphere.

Or, may just have been a draft ;-)