Fortresses are completely impassable and give no terrain stars. They cover twelve squares, in a 4x4 square.
Usually, but not always, they are protected by either four or eight Minicannons. They are the focus of three Campaign missions: Show Stopper and Sea Fortress in Advance Wars 2: Black Hole Rising and Omens and Signs in Advance Wars: Dual Strike. They also make an appearance in the Surrounded! mission and various War Room, Survival and Vs. Mode maps in Dual Strike, but seem to serve no purpose other than decoration. In order to be rendered inoperable, the surrounding Minicannons have to be destroyed.