FFVI - Walkthrough - Cave of the Sealed Gate
Imperial Base
If you enter without Terra you'll be kicked out. So if you don't have Terra return to the airship and change party members. In the building is a treasure room with a locked door. Make your way thru to the other side of the base and exit. Head east and you'll enter the Cave to the Sealed Gate.
Cave to the Sealed Gate
Chests: Assassin, Tempest, Coin Toss, X-Potion, 3 Ethers, Genji Glove, Tent, Inviz Edge, Water Skean, Remedy, 2000 GP, Elixir, 3 Magicites, Atma Weapon
In the room after the one with the moving bridges, pull the switch on the bridge. Now follow the left path to reach a chest with Genji Glove. Now go back and take the right path. Go up and pull the left switch. Enter the doorway that appears, get the chest, and save your game. Pulling the right switch makes you fight Ninja. Pull the switch on the next bridge and go down to the stairway it makes. Search the ground in the area that is below the stairway, there are 4 hidden treasures: 1 to the left of the stairs, 1 a couple steps above the hole going right, 1 a couple steps to the left of the slope that leads up to the chest, and 1 a few steps above the chests. This is the treasure that the Ninja refers to when you defeat him. Go through the hole after you get the chest and hidden stuff. Step on both switches you can see. 
Go down and then left. Step on that switch and enter the door that appears. After getting the treasure leave the room and go Screenshot right to the far bridge and then up. Pull the switch, cross the bridge, and pull another switch. Go up and left across the brides. Flick the switch inside the chest and return back to your right. Grab the chest up the stairs then go down and exit the room.
Go up the stairs in the next room and your at the Sealed Gate. Now you have to fight Kefka again (kinda). To get back to the airship all you need to do is go down and out, and then go back thru the Imperial Base. On the airship the Espers attack and you lose control of the ship, crashing just south of Maranda. From here go northeast to Vector.