Thanks for the replies everyone
The problem with capturing castles/capitals for me is that I have an old pentium 4 and the castle sieges lag horrendously for me, and the battle size slider is already all the way to the left and it doesnt seem to matter. There arent supposed to be more than 14 troops on the field at once but there are regularly much more than that in siege battles.
Thats why I cant capture any castles, all I can do is loot villages.