It takes place on the roof of a Castle, which is being attacked, hence the name Castle Siege. The stages' main platform is small with one side elevated more the other, and it has a few extra platforms suspended in the air by the towers behind them. However, after a certain time passes, the roof fades away, and the players will be in the Castle's throne room. This area has walkoffs. It has a few platforms, held up by statues, that can be destroyed as well as . This results in the platforms falling, thus less places to run. After a certain time passes, the floor of the throne room disappears and the stage transports the player to an underground cavern. The underground cavern is a dark lava filled stage, looking similar to Norfair.
Castle Siege is the first Fire Emblem stage in the Super Smash Bros. series, as Super Smash Bros. Melee didn't have a stage representing Fire Emblem. Instead, in Melee, the special events that starred Marth or Roy took place on the Zelda themed Temple stage. This particular stage also featured music from Fire Emblem if the player pressed and held down the R button while selecting the stage.