OP your tip lacks certain things. For one, engies can't spam level 3 sentries. It takes time to build them, and they can only build one (thanks patch). FaN doesn't spam, it only has 2 shots. Your opinion on the maps isn't a gameplay tip. "Getting behind enemy lines" is a strategy, and isn't required solely for Watchtower. Trying to assume what will happen if we play exactly how you play isn't much help either, because there are never scouts that kill me with scatterguns and there is never a camping pyro for me. You told us to use spy for watchtower, if everyone uses spy than your team sucks. And if you are spy, then you shouldn't have problems getting through scouts and pyros (contradiction yus?). Demoman has a serious advantage on Watchtower, why would you not want to play him? His stickies allow maximum maneuver ability around the rather tall in height map, him solder and scout are the only ones capable of getting on the roof of the tower themselves.
Wall of text/rant activate!