Welsh weather is pretty fickle too, particularly on the Western fringes. I wouldn't rule out colour film entirely, mind. Conwy is lit by night too - see http://www.uk-photo-library.co.uk/wa...wy/pages/6.htm
Some links you might find useful:
A site covering all the Welsh castles in detail (and Abbeys too):
Wales' official heritage body that is charged with looking after most of them:
The homepage says there is free entry to 12 of the castles on 1 March, St David's Day (Dydd Dewi Sant).
Conwy and Harlech are two that I know are near train stations. More info from http://www.arrivatrainswales.co.uk/
BTW check your spelling before you try to garner info via searches e.g. Beaumaris (I think there's a bus service of some sorts from Bangor to this one). Local Tourist Information Centres will be able to assist too. Official tourism websites: