bActAsCity will also make ships able to enter coastal forts. This is normal BtS behavior, but some people thought it looked odd and complained.
The main problem with forts acting as cities is that the AI will try to use them to collect mana resources instead of building the proper mana nodes. This is especially a problem when they connect Raw Mana with a fort instead of using a node to transform it into a useful form.
In FfH Forts upgrade to Castles and then Citadels over time, whether worked or not. The worker AI does not consider what the upgrades can connect, only the base improvement. Because of this you can avoid the mana issue by making castles and citadels connect resources but not unupgraded forts.