Koopa Bros. Fortress

Appearance and description
The fortress is based mainly off of the castle levels from classic Super Mario games. The background music is also similar.

The fortress itself is near what appears to be a running river.

There exist cracks on the walls in some rooms (which adds usefulness to Bombette's abilities), and said rooms are loosely guarded by enemy red-shelled Koopa Troopas, Koopa Paratroopas, and Bob-ombs. On the roof, during the events of Chapter 1, the Koopa Bros. order their Bill Blasters to launch an array of Bullet Bills at Mario as a final line of defense. They do not reappear should one revisit the fortress' ceiling.

Background
According to the map section of the Pause Screen in Paper Mario, the fortress was originally owned by Bob-ombs (it is unknown whether or not these Bob-ombs were Bowser's followers or if they were Koopa Village natives) until the Koopa Bros. were assigned the job of taking it over.

Once Mario defeats the Koopa Bros., they are temporarily trapped in their own jail cell with their Bowser replica. They are later freed by an unknown being (or, they may have escaped themselves), hence their plot for revenge (seen in the letter sent to Mario's mailbox at his house) and reappearance at Bowser's Castle. The fortress is apparently currently abandoned.