Castle Siege is a Fire Emblem stage from Brawl that was brought back in Smash 4. It's a generic castle that isn't really from any Fire Emblem game, like the Coliseum that succeeded it. Castle Siege is split up into three acts. The first part takes place on the wall of the castle, as seen to your right. Despite a massive battle taking place in the background, none of them can hurt you. They're probably really weirded out by the fact that a penguin with a giant hammer, a racer who wants you to show him your moves, a two-dimensional LCD guy, and a yoga instructor are smacking each other around in the middle of their fight.
After a while, the stage will transition into the castle, where some king guy, I dunno, let's call him King Gorchaf or something, is sitting at his throne. He's got his army of soldiers ready to strike you at any moment! But not really, they're all cardboard cutouts. This section also has giant statues for you to break, in case you are in need of a little catharsis.
After that, though, the stage transitions again, this time to the underground of the castle. The Smash Bros Dojo didn't say anything about this part, telling you that "You can look forward to finding out later." Sakurai: *laughs*. However, I'm not Sakurai, so I'll tell you. It's a big platform in the middle of a gigantic lava chamber. Not much really happens here, but it looks cool at least. And then it transitions back to the castle wall from the beginning, restarting the stage all over again. This stage is pretty cool, I like it.
Even though the stage isn't based on any single location, the ascetic and music alone have given Tellius more love in one game (or two if you count when it returned untouched in SSB4) than the Fire Emblem series has in nearly a decade.