How to deal with hordes of longbowmen?

I thought I knew a lot about the game but then I found this sub and reading some posts there is a lot more information than I previously thought.

I play through voobly with a group of about 3 or 4 other friends and recently, one of them discovered the Britons and their castle unit. The longbowmen. He uses them to disrupt trading routes and they absolutely destroy my army due to the fact that they can stand so far away from the actual battle.

It would be very helpful if anyone had any suggestions on how to get around this. Thanks!