Your walls/bridges across the river could look like arched Roman-style aqueducts but instead of transporting water, they'd let people across. Then you could drop pitch or rocks on top of any intruding boats. Narrow-spaced archways could then only allow smaller boats and pretty much deny anything other than canoes or fishing boats. Of course this could only really be depicted in ISO or profile views but you could do up a shadow that shows the arching. Catapult stations are a good idea too, especially on the high cliffs, as are the chains. Oh and by the way, this looks outstanding so far and as SG said, the colors are very sweet.