I use castles for Ulm. Ulm's primary units can be recruited at any fortress as can their mages. The castle allows for greater overall production than the FC...you can build castles faster and closer together without restricting unit production in provinces with 2 adjacent castles. The only reason I would use an FC for Ulm is in the manner I use them with the other Capitol-only unit nations--to pump out Black Templar in Iron Faith, but I dislike Iron Faith....
As for Watchtowers being a "weenie" tactic, I disagree. I take the watchtower when I want to focus on magic--I can hire more commander units with a watchtower because I can build them right next to each other. Why should a player who has no intention of pumping out huge armies of troops have to take a "non-weenie" castle? If I take Sloth-3 for some reason, a bigger fortress is simply a waste of design points. What about the Mausoleum? Is it for weenies or does the 40 design point cost allow you to play like a weenie. Watchtowers are VERY easy to besiege. Teleporting armies can be a problem, but you shouldn't step unless you can represent
