Another strategy, the one i use. Requires having about 6-15 melee infantry to plug the 2 small entrances, 10-30 Calvary, and 2-3 singulares, with the rest being marksmen.
You leave the Marksmen in the fort, while forcing your Calvary and singulares to follow you away from the enemy and fort. As the enemy begins making its way closer you force your Calvary to charge around the enemy's infantry straight into the marksmen leaving you and 2-3 singulares to roam killing from horse back. (i typically use my small elite group to break up clusters of marksmen).
I have used this strategy with anywhere of 30-87 men against 2 to 3 times that number successfully.