I like your ideas for the Welsh tech tree. Just a few things to maybe think about.

Dont produce celtic troops like in MTW as they were a bit more advanced than running round naked as if they hadnt progressed since Rome invaded 800 years before. Crude leather amour for the basic sword and axemen and maybe chainmail for the more advanced troops. The Welsh werent really technologically advanced enough to make large amounts of plate armour and the style of warfare that was implimented didnt suit heavy knights.

As for the city walls i think that they should be able to build castles that match those of the Normans. Although most of the castles left standing in Wales are of English origin, you only need to look at some of the castles built by Rhodri Mawr to see that Welsh Castle design was as good as their neighbours.

Last thing, what about an elite Welsh unit like the 'Welsh bandits' in MTW. In Welsh warfare most of the troops on the battlefield used the bow before a mad rush from all sides due to their ability to ambush, can this be represented somehow??? As for those elite troops, perhaps a mastery of the longbow, ability to hide anywhere, and very high melee skills using a sword? and also maybe an elite longbow units that can hide anywhere aswell.