Southern Fort is the third stage in the Dark Descent Stages.
The stage starts off spawning Doge Darks at a medium rate. When the base health reaches 97% of its HP, 3 Gory Blacks spawn along with Doge Darks as supporting peons. Nothing else comes out of the enemy base other than Doge Darks when hit again.
This is a very easy stage. Make use of any meatshields and anti-blacks at your disposal and you should win.