Yes and yes. Being an old fort, there are some stairs so a wheel chair would not be able to go everywhere. BUT since it was a cannon based fort, they have ramps everywhere that were put in place to allow the cannons to be rolled between the fort's levels, so (while they may be pretty step) a wheel chair can see a good chuck of the fort.
Yes, its all walking. In fact you can walk to te fort from the ship (its less than a mile)
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007