Those tower icons are actually watchtowers built along the city wall, roughly one mile apart from each other. Any Gold Cloaks stationed on the watchtower will be able to observe the field several miles out, giving them an early warning when the enemy approaches. Considering this scenario is set to 0 theater recon for both sides, the watchtowers are quite a deal.
The rail serves two purposes. One, it confines the movement of "Great Chain" unit, represented by rail artillery counters and equipped with armored equipment, to that small area of a choke point at the mouth of the river; second, it gives both sides a visual hint as to where the chains lie initially underwater, as described in the book (the Baratheon fleet noticed chains underwater, but did not react to it, and was later cut off by raised chains). They cannot be traversed by a ground unit, because of the deep water terrain feature.
Here is a picture of great chains raised. When raised, they cannot be destructed by ships, denying passage to the enemy fleet.
< Message edited by Annabelle1993 -- 3/10/2018 2:14:11 PM >