When the match begins, players begin the fight on a small section of the unnamed castle's roof, which is bombarded by heavy fire from catapults. Eventually the floor shatters as the castle defenses waver and players fall into the throne room, which has breakable statues, four platforms and a solid floor with no drop-off points. Two platforms are held in place by destructible statues, just like Luigi's Mansion, which means that this could cause problems for projectile usage. There are also two hanging cloth emblems above serving as platforms. Players later fall again into the underground, which contains one stone platform balancing on a stone spire in the midst of a lava cavern. A part of this area is slightly elevated. After a while the stage will transition back to the top of the castle.
The "changing stage" effects of Castle Siege occur at regular intervals. For example, the initial siege setting lasts until approximately forty seconds into the match. These changes can have the effect of saving a character who would have been otherwise unable to recover. This is particularly useful for Bowser players attempting a stalled Flying Slam or Ganondorf players attempting a stalled aerial Flame Choke, among others.
This stage is usually a starter, but is sometimes a counterpick because of the statues blocking projectiles in the second level, limiting approach options for characters like Mario and Falco. The second level also has walk-off edges, allowing easy and/or early KOs, such as a high knockback attack being able to finish off an opponent at stupendously lower percentages than normal when near the blast line, or characters with chaingrabs that involve walking or dashing being able to drag an opponent past the blast line (as with all stages involving walk-off blast lines), such as King Dedede with his down throw.
Throughout the entire Fire Emblem series the main objective for many chapters was to seize a castle, gate, or throne. This is more than likely where the name of this stage came from. In the official artworks of the Fire Emblem games has depicted castles as being immense with lush environments around it. The castle of Altea is also depicted in the Fire Emblem anime. This stage in Brawl has a castle that designed after various castles in the Fire Emblem series' artworks and the castle also has some resemblance to the one in the anime. The castle is also surrounded by lush environments. Another motif in the throughout the Fire Emblem series is that most of the villains are made up of dragons. The symbol depicted on the flags in this stage is a dragon. In three Fire Emblems a weapon called the "Pachyderm" (aka an Elephant) is a weapon which fires fireballs at opponents. In the background of this stage can be seen the fireballs of somebody using a Pachyderm. In Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn an event known as the "Serenes Massacre" occurs decades before. The ruler of Begnion, Misaha, was assassinated. Begnion Senate put the blame on the Heron Tribe, which was false because the Herons prefer to live in peace and have no knowledge of fighting skills. The Herons also lived in the Serenes Forest. This accusation convinced the citizens and a large number of citizens got together at the edge of the Serenes Forest and burned the forest down with their torches. The fire lasted for three days and only five Herons survived. This stage in Brawl shows a forest fire burning in the background, which could be a reference to the Serenes Massacre.
Most of the chapters in the Fire Emblem series happen indoors where you're trying to seize the throne that is usually guarded by a powerful boss. The Black Knight is one of the main villains, who is in black, heavy armor. In the indoor part of this stage can be seen the Black Knight in front of his throne in the background. Both Path of Radiance and Radiant Dawn have soldiers that can change their class to "Halberdier". Those Halberdiers can be seen in the indoor part of this stage in front of the Black Knight in the background. Fire Emblem: Seisen no Keifu was the first Fire Emblem to feature a background during conversations. One of the backgrounds resembles the entire indoor room of this stage. Fire Emblem: Fuuin no Tsurugi was the first Fire Emblem to have destructible terrain. For example, a wall has a certain amount of HP when the player attacks the wall enough to where the HP reaches zero the wall crumbles. The statues in this stage could be a reference to the destructible material.
In Fuuin no Tsurugi was an extra chapter which is a magma stage. Ever since Fuuin no Tsurugi, with the exception of Path of Radiance, there has been a magma stage. The last area of this stage in Brawl features a magma stage. 
- Castle Siege is the very first new stage introduced in Super Smash Bros. Brawl.
- The Pokémon Trainer stands in the little parapet on the right when he's on the first tier of the stage; if there are three or four Pokémon Trainers, two will stand in the little parapet on the right, while the others will stand in the little parapet in the background.
- Though not officially from any one Fire Emblem title, the stage sports flags similar to that of Daein, the antagonist army from Fire Emblem: Path of Radiance. In addition, Halberdiers, who are unique to the Radiance saga, can be seen in the background of the second tier.
- During the throne room phase, someone who bears a striking resemblance to the Black Knight can be seen sitting on the throne in the background.
- Pausing the game while the stage is going through a transition will end the transition quickly, since the game continues to load the stage as the game is paused. A similar effect happens with special moves that involve character switching.
- Sometimes there are signals which shows that the stage will transform. In the first stage, all the fire balls in the background will stop. The next stage, all the torches in the background will blow out.
- If a match on Castle Siege goes into Sudden Death, it will begin on whichever tier the match ended on, not necessarily the top level. This is likely done so the game doesn't have to reload the top level before Sudden Death.
- Stage confirmation
- Music confirmation
- The Fire Emblem Wikia
- Castle Siege in Smash History (TrailerDrake)