Apparently you need to unpack Kingdoms, see
cant install really bad ai
It is not the Welsh who are retarded but the battle and campaign AI. In my view the Kingdoms Crusades campaign and battle AI are better than Britannia, as well as better than Vanilla. I played Britannia as the Welsh and conquered all of England without the English making a single move on me after turn 14-15. Even if they had a massive army next to a castle guarded by a single unit they would not lay siege on it. Moreover, when the English king returned from his Crusade with a nice army full of Templars, he stayed in London alone and the AI took the army without a general for a walk in East Anglia. The result was that the Templars were massacred and a few turns later London was captured with the king trying to defend the city on his own with his bodyguard.
Based on that experience, it is even surprising that in your game the Welsh actually went into the trouble of besieging something.
What is ridiculous in my view is that Creative Assembly have made those extremely hard to win historical battles, you spend weeks trying to win them, you feel like a genious for winning just once, then you start a campaign and the computer player cannot even put a decent army together, let alone fight sensibly. It can obviously be done, if Creative Assembly could give you a hard time in the historical battles.