After you defeat the Doom's Wall and Vampire in the Empire Ruins, the Emperor wants to make a truce in Northtown Castle.
Go to the Castle
Use the northwest exit of Northtown to reach Northtown Castle. Go north and through the center door. In the castle, there's only one path that you can take, because soldiers are blocking the other paths.
You'll reach a dining room, but it's mysteriously empty. Go to the north end of the room and talk to the guy wearing the horned helmet. This is Emperor Vandole, and, as you might expect, this was all a trap! His plan all along was to get rid of both the Resistance and the Mana Knight.
You'll end up in a cell with Krissie and the other Resistance Members. Talk to the soldier through the bars. The soldier lets you and Primm and Popoi out, but not the others. Then he dumps you into a pit with Metal Mantis.
Metal Mantis
It's weak against Thunderbolt. Use that repeatedly until it dies. It shouldn't take long.
After the battle, you get a Glove's Orb. Step on the checkered tile that appears in the middle of the room after the battle.
Escape
Go west and step on the yellow creature to go across. Follow the path and go up the stairs near the end to get 1000 GP from the chest. Then go through the door in the north wall.
There are Embermen in the next room. They can replicate themselves, and they can also cause you to become engulfed in flames. They are pretty easy to defeat, though. Go up the stairs to the next room.
Step on the switch to the left to free the Resistance members. All of the Imperial soldiers have left for some reason. Go west and up the stairs. Get past the Embermen and use the door in the south wall.
Outside, there are some Armored Men. They aren't too hard. Go all the way to the right and through the rightmost door. Go up and step on the switch that the Armored Men are guarding, then go north through the door and get the Axe's Orb and Whip's Orb from the chests. Be warned that some Armored Men appear after you open one of the chests.
Go back outside, down the stairs, and into the big main door on the lower level. There are some Armored Men here. Go north up the stairs. There's a Dark Ninja up here. Go up the northwest stairs.
Follow the straightforward path until you reach a room with some tables. Go up to find a couple of Dark Ninjas hiding behind mirrors. Go to the northeast corner of the room and step on the switch, then go through the door that appears.
Follow the path until you reach an area where you can either go right or left. There is a dead end to the left, so go to the right. You'll pass by some beds, then you'll reach some stairs in the northwest corner of the room. Go up.
In the next area, there is a Dark Ninja. Go to the right, then follow the path to a warp tile. (The warp tiles below will just knock you back.)
In the room where you warp, go through the arch to the left. Deal with the Dark Ninja. There is a warp tile to the right, but it will knock you back. Hit the switch on the wall just to the left of the warp tile. This will deactivate the trap so you can walk over the warp tile. Go up, prepare for a boss battle, and go through the double door.
You'll go to the back and find the Emperor surrounded by his henchmen Sheex (in purple), Fanha (in red), and Geshtar (in green). Geshtar asks for another chance to defeat you. Emperor Vandole grants his request. Geshtar becomes Mech Rider 2.
Mech Rider 2
He has no weaknesses, so use any spell. Air Blast is a good one, or Thunderbolt. Hold the A button to run to the left or right to quickly reach Mech Rider. If he runs out of MP, he'll use Wave Cannon, which causes a lot of damage.
When Mech Rider 2 is defeated, you get a Javelin's Orb. Geshtar decides to destroy the castle and you along with it. Just when all seems hopeless, you hear a strange drumming sound, and the voice of King Truffle. Go to the right and talk to King Truffle. Apparently Flammie has grown to full size and can fly, and he brought King Truffle here. Flammie thinks you're his parents. King Truffle gives you the Flammie Drum, which you can use to summon Flammie anywhere. Now you can fly wherever you want! No more Cannon Travel!
King Truffle asks you to have Flammie fly you all to Matango, which is to the southwest. To fly, press left or right on the directional pad to change the direction that Flammie is facing. Then press up or down to fly forward or backward. Aim Flammie southwest. Fly across the ocean and then look for a kingdom at the base of some mountains to the right. There are a bunch of blue and golden things on the ground south of this kingdom. To descend, hold B. To rise, hold A. Hold B to land in Matango. (Note, there are some places where you can't land. If you don't land after holding B for a long time, move to a different spot and try landing there instead.)
When you land, Truffle will be standing outside of Fung Castle. Talk to him. He'll mention that you can press Start to see the World Map while flying on Flammie, and press the L or R button to change modes.
Truffle says to go to the Lofty Mountains southwest of Matango.