How to counter trebs Protected by units ? ( Next Castle )

Hi guys who do you solve this ? I have played few games where this happend
I could find any reasonable answer how to counter situation where unit are protecting the Most frustrating was that he was the game in black forest oponet was Britons in corner pretty much losing surrendered by my krepost and konniks and skrims . He was Britons they hand longbows I just could not get to them .

Thanks for help Aoe2 community !