Borderforts work best and sometimes you'll have to garrison a town militia next to it to see even further.

Something I noticed ifyou sit next to an enemies border fort you can use it.

I always zoom in look for the highest ground and place a border fort there this gives the best site radius for a border fort. Usuall up north is where border forts have problems with all the trees but if you garrison next to it, you will be able to see into the trees