The true first "level" of the game would happen at the Castle's bridge. Tons of incoming creatures in the foreground and background trying to leave the castle, while Julius fights his way through it in sidescrolling action. He's stopped midway by a Final Guard that takes lot of punishment but never dies. It gets destroyed by a missile shot by an helicopter. This helicopter goes on to give Julius cover as he walks on the bridge and you can see soldiers fighting the monsters in the background. Finally, the helicopter is taken down by a sea monster (which would be one of the bosses of the game, and not a serpent-like creature
).
That's where I'd show that, whenever you try to shot missiles at the castle, HORDES of flying demons (bats, gargoyles, devils, imps) sacrifice themselves to protect it by flying against the missile and dropping it on the sea or flying it up making it miss the target containing its explosion, as antibodies protecting the main body.
When Julius broke through the main gate, that would be the first boss in the game. After defeating it, Julius would STILL have to deal with the main corridor's entrance swarming with demons that can craw all over the walls. After reaching the first save room, Julius would find a hidden mechanism allowing for him to close the main gate's bridge (temporalily) until the endless swarm of monsters stopped coming out at such a consistent pace. This is where the game would slow-down enough progress the story at metroidvania pace.
The music track for this scene would be a fast arrangement of Super Castlevania IV's "Into the Castle/Ascension to the Castle", something like this, but more orchestrated and less metal:
www.youtube.com/watch?v=HV5k6DgBqGkI say "true first level" because the game wouldn't start here for real. Before that, the player would control Julius in his last training with Jonathan (where all the controls and combat techniques would be taught to the player, much like Barlowe teaching Shanoa), and would again control Julius in a "real" combat situation in another thing, before actually going for the war.