Oda's Castle, or rather, Japanese castles aren't really different from Western ones. If you insist on distinguishing them, we can say Western castles are more streamlined while Japanese ones are squarer. But there's one thing that's always the same: the boss is always at the highest place.
If it were a typical invasion, the invaders would have to get through moats, bridges, break through the main gate, then fight their way into the castle, continually battling countless enemies, and gradually struggling to the top... essentially exhausting themselves in the process.
The approach of Duanmu Huai is much simpler.
Why not just parachute in and smash through the ceiling?
Skipping all the other steps entirely.
Of course, it's not like everyone else could parachute in like Duanmu Huai, but luckily, Duanmu Huai had thought about this beforehand.
"Um… what is this??"
Looking at the thing hanging on the edge of Sky City, which looked like a large metal box, Ling'Nu blinked curiously.