Border Mulan is an area on the border of Renais and Frelia. It is some distance northwest from Renais and is a small area with some mountains and a castle. In the game this is the first chapter after the prologue, Eirika and Seth fight Grado soldiers and rescue Tana, who had bee captured while waiting for reinforcements.
The location is possibly used as a regulation point for travelers but nothing is elaborated as it is simply the map for the first chapter of the game.
Monsters never spawn at this location during the Creature Campaign or main story.