I love having more fort options! Ahwaric has done a lot of great work on forts if you wanted to borrow his Orbis code, as well as do some play testing from what he has already done. One of the good things about Orbi forts is they grant cultural borders if in the wild. I like the idea of area around forts limiting movement to 1 square per turn (except for flying units). As well as the a. b. c. options you listed. One other option I would consider is a garrison fort ability (available at warfare?) - allowing units to garrison a fort for less upkeep (or no upkeep) cost. Thus giving incentive to put units on the front lines. Another idea is create early forts (available at crafting) like "peel towers" and have workers "work" them to become forts, keeps, castles. the ability to work the tower tiles can be given at construction or another similar tech. Because keeps and castles upgrade from ancient towers, I think forts, keeps, and castles should keep the extra visibility range. One of the main purposes of defensive structures was to give early warning... and that also stops the micromanagement of using hawks for all your cities (which gets annoying). A possibility for reducing movement around forts is traps (castable by units garrisoned in the fort, or fort commander). Traps could be made like grasping vines in that they can't move. Since attacking takes a movement it will slow an army. A combination of reduced movement and traps would be really deadly. I should stop here... I have too many ideas for forts because I would like to see them as a big strategy part of defense.