Ok..
here is how I eliminated heros being built from "fortresses".
1. Select a building to be the fortress.. pretty much anything you want. - If you want your map to depend on the fortress' survival to win or lose, name it player#fortress, and assign it accordingly.
2. Select a WOR fortress (real, recognized fortress) for the same player.. and place it way off the map (this is to allow Ring Powers)
3. Place your player 1 start position on the map in front of the fortress you placed ON the map, and have that start position be a BEACON instead of the normal.
As for making the FORTRESS be the determining factor for win or lose.. simply script it as such:
KILL PLAYER:
if PLAYER#FORTRESS has been destroyed, all PLAYER#'s buildings and etc are dealt a lethal amount of damage
I know, it sounds complicated, but to actually do it, it is very simple.