The Battlefield Fortress is a level in the Subspace Emissary. It appears as a large castle surrounded by a desert.
At the beginning of the stage, there is a cutscene which shows two R.O.B.s activating a Subspace Bomb. They hang their heads in sadness as it detonates. From atop the castle, Marth sees the explosion and seems shocked by it. Soon afterwards, an army of Primids approaches the castle. Marth raises Falchion and prepares to defend the castle. After defeating the foes and leaving the castle, Marth stares at the sphere of Subspace caused by the earlier explosion, when Meta Knight appears out of nowhere and attacks Marth. After an evenly matched sword duel, each one sees Primids over their rival's shoulder. They dodge around each other and destroy the Primids with their swords. They turn to face each other, surprised to realize they have a common enemy. After more Primids surround them, they team up to destroy them.
After another level that involves destroying more enemies, Meta Knight and Marth see the Ancient Minister carrying another Subspace Bomb. They both attempt to destroy it, but fail. Suddenly, the yell, "Great Aether!!" is heard, and Ike appears, using his trademark Aether to slice through the Subspace Bomb, destroying it. Marth, Meta Knight, and Ike all then team up to take on more enemies from Subspace. After doing this, the stage is clear, and the swordsman trio is added to the player's party. The final cutscene involves the swordsmen giving chase to the Ancient Minister, but he is able to escape.
After the level is complete, an entirely different cutscene proceeds. In the cutscene Luigi is seen timidly confronting a Waddle Dee. When the harmless creature naturally avoids confrontation, Luigi is relived but shocked by the appearance of another Waddle Dee. But as he prepares to "fight", King Dedede appears behind him and ambushes him with his mallet; turning him into a trophy. King Dedede then notices Wario and his cargo containing the trophies of Peach/Zelda and Ness approaching. Using the Luigi trophy as bait, King Dedede is able to steal the cargo and the trophies much to the chagrin of Wario.
- During the end of the opening cutscene, a small clip of Fire Emblem Theme plays, in a different key though.
- During the first part of the stage where one plays as Marth, Preparing to Advance plays.
- After teaming up with Meta Knight, Meta Knight's Revenge plays.
- After the fight, Attack plays.
- During the underground cavern section, Crimean Army Sortie plays.
- After teaming up with Ike, Ike's Theme plays.
- After the level: When Luigi appears, Luigi's Mansion Theme plays, but in a different key.
- During King Dedede's first appearance, the very beginning of King Dedede's Theme plays.
- Marth Atop The Fortress
- Marth Encounters Meta Knight
- Ike Unleashes Aether
- The Three Warriors Chase The Ancient Minister
- King Dedede Steals Peach
- King Dedede Steals Zelda
- Oddly, Ike yells "Great Aether!!" while performing just a normal Aether on the Ancient Minister during the third cutscene. This is because the voice clip of "Aether" in Japanese is said in two parts (天空 Ten-kū, lit. "Sky"), so the voice clip of "Great Aether" was used instead on the English versions.
- There is a very obvious design flaw to the fortress: the button to open the gate is on the outside. Luckily, Marth can poke his sword through the door to press it. This switch does not need to be pressed when the stage is replayed, as the player could potentially choose a character that cannot press it.
- On the Adventure Map, it is revealed that The Battlefield Fortress is right next to the Forest.