I've built all of one watch tower in this game and have not found them useful for anything once you get to building stuff that tower over them. I think the best solution would to have a late/mid-game upgrade for these to include a way to route a power cable and at least one conveyor pole through them.
The cost for these should match the total cost of the tower + power pole + number of conveyor poles as if they were created individually. Pepper in some extra cost or additional resource to make them.
The gain would be that power and conveyors routed between towers get a reasonable distance boost. This would allow players to expand their bases and cover distances cleanly in a single construct without any unnecessary complexity.