Traverse Town 2

When you arrive, go to the Alleyway at the Second District and activate the Red Trinity mark in the water to access the Secret Waterway. There, go north first to get the chest with Puppies 10-12. Talk to Leon twice and he'll give you the Earthshine summon gem. Aerith will suggest that you go show the Navi-G piece to Cid. Go back to the First District and talk to Cid. He'll install the Navigation Gummi and also give you the Old Book. Proceed to the Magician's Study and give the book to Merlin. You will also receive the Simba summon from the Fairy Godmother.

Go back to the Third District for a cut-scene with Riku, then proceed to the Small House afterwards. Here talk to Cid again, and there'll be a short cut-scene between Riku and Maleficent. You'll get the Warp-G from Cid.. Aerith and Yuffie also says something about a bell behind the Gizmo Shop.

Now go into the Gizmo Shop and exit through the north door. Climb the ladder up and you'll see a Red Trinity mark -- activate it. Pull the rope 3 times to reveal a keyhole. Go back to the Accessory Shop and save your game. When you're ready, approach the keyhole for a boss battle.

Boss Battle - Opposite Armor

Just like Guard Armor, this one also has a lot of parts. Go for the feet first, then the body. Also, destroying his parts yields HP balls. When the body starts shooting a beam at you, just roll out of the way to avoid it. Heal whenever you take serious damage. Easy enough.

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After sealing the keyhole, you'll get the Aero magic and another Navi-G Piece. Take the Navigation Gummi to Cid if you like, but he'll simply say that he needs the other pair to install it. There's nothing left to do here other than to stock up on items. Board your gummi ship when you're ready.

Take the warp hole that's located directly above Traverse Town and fly to Agrabah.