Isn't the fortress just an aluminum danforth?
No. First difference is that the fortress has two settings, one for soft bottom and one for hard bottom. The difference in settings is the angle in which the flukes are. I believe there are other suttle differences that may help but I can't remember what they are. I did compare it to my old danforth when I got it and did see some mild differences.
The other great difference is that you can buy a real big one, take it apart and stow it below to use on a rainny day

Or you can put it on the bow and it still be lighter than the danforth but it will have much greater holding power. It will however, cost a lot more
Many say the fortress does set better than the danforth but I bet that both should be set the same way...get it to dig in with a short scope so that the tips are pointed down into the ground and the back of the anchor is raised off the ground, then once it's catching and digging in, then let the scope out and put a lot of pressure on it (back down on it). I do believe that is the trick to those anchors.
Now don't get me wrong, my fortress is not my primary anchor, it's my storm anchor. I like my primary anchors to be ones that can reset themselves on wind shifts in the middle of the night. If the wind shifts, the boat isn't going to reduce the scope to 3:1 to get it to dig in for me.