
Basically, each platform would hold several vehicles or animals, move at walking or running speed and as one platform moves to the other side, the other platform moves to the side that the player left. There will always be a platform on either side, unless it's moving. Also, the ropes could be underwater and have no hitboxes, allowing boats and knarrs to travel over them. Not sure if it would work in the games setting, but it would solve the problem of river crossings without the issue of impeding water craft. I already posted about this in the new world prelude, but I wanted to add a topic in C&I, too.
It would cost a fair amount of wooden blocks, possibly planks, bone glue, metal(?) and an amount of rope relative to the distance it is crossing.