Labatt...dog has it right. The fortress gives you a 45 degree angle setting that is perfect for mud and the fluke area is larger and the anchor itself is significantly stornger. It also comes with "mud palms" which help even further to dig down into the mud quickly.
I am not suggesting everyone needs a Fortress...as I favor some of the newer designs as a primary...but in this particular type of bottom and tight anchorage it would be the best to have IMHO.
Check out these test results in mud vs. danforth: