Instructions
Walk along the stony seaside path after chapter 5-1 loads. As you get to a gap in the path, a short scene will show Ashley being kidnapped by Ganado monsters.
Fight through the waves of monster people in the fortress area. You must kill every Ganado to advance, but pick your attack methods carefully. Shoot the Gatling gun-wielding J.J. Ganado from a distance if you can, as he will mow you down in the open. Other foes can be lured to the edge of cliffs to be pushed off with a single bullet.
Check the main door of the fortress, and the game will hint that the laser beam emitted from it is the key to entering. More Ganado will appear with cattle prods and wooden shields, so be ready to fight.
Follow the laser to the roof of the building with the golden reflecting mirror. Climb to the roof and direct the laser's beam to the mirror on the other side of the fortress.
Climb the ladder to the building with the second laser-reflecting mirror. Direct the second mirror's beam to the orange rectangle beside the fortress door. The main door will open, allowing you to continue the adventure. Be ready to avoid a boulder shortly after entering!