This can be done any time after getting the Cloak Ball from Babi. Go through the Gondowan Cave to reach Angara from Tolbi. You will emerge near Kalay; heading north across several bridges, past Vault, should take you to Lunpa. Do not enter the town proper, but instead the cave next to it.
West Lunpa Cave
A short cave you'll need to pass through to enter the town, which is currently under lockdown. Go north, and when the path splits, take the right one. Soon, you will come up to a door which is stuck partially open and a water puddle underneath; Frost it, and the ice pillar will force the door up. Walk through and around the wall to enter the town.
The town is locked to outsiders, hence why you had to sneak in through the cave. Due to this fact, the inn is closed and the shopkeepers have no goods to sell. However, if you do need healing, use Reveal on a stone circle in the southern left corner of town to see a Psynergy stone. In the north of town is the passage to Dodonpa's headquarters, where Hammet is being held. Before going there, use Reveal on a rock in the passage to see a hidden water puddle. Carefully, Frost it, then climb up the ladder (without walking outside of Reveal's range) and jump across. Head south down along the cliff, across the gate of town, and use Reveal when you seemingly reach a dead end. Examine the glowing light to show a doorway. Enter, and inside are six treasure chests containing several different kinds of items. Now, exit the room, and slide down. Go north into the Fortress's gate area.
Once here, walk to the left wall and use Cloak. This will hide you in a shroud, just like when you first saw Babi. You can (very carefully) walk behind the guards using this technique; be aware that if you step into direct light, you WILL be revealed. Enter the fortress now.
Inside, head through the first door immediately. Note that the area is very similarly lit to Altmiller cave; there are torches on the wall that spread out in circles. If you enter the area lit by a torch, you will be revealed. In the door directly ahead of you are several henchmen, as well as a few servants. Talk to them if you wish; in the main hall, go down the right path and head north. Use Cloak to hide from the green men, who will throw you out if they see you. Go through the door, staying out of the light as long as possible. The next room, head north around a wall to stay out of the guard's sight, going into Cloak whenever possible. Once past him, go through the door.
This room has a walking guard. Wait for him, in Cloak, to go as far left as he does, then quickly dash by heading south and through the door. In here, use Cloak immediately, then run south and up the stairs. There is a guard pacing very quickly back and forth, but he doesn't come any closer. Walk right, then go through the door. Once entering, you will see a red guard this time, instead of the usual green. As soon as you are in the room, you're already in the light; Cloak is impossible to use. Walk by, and he fights you. He's very weak, so don't worry about it. After he is defeated, you can go north up the path he was formerly blocking. In here are Dodonpa's wife and kids. Talk to them if you want to, then return to the room where the guard was.
Go back one more room to the area where the two guards were pacing quickly; use Cloak and try to run past him, then head right into a small area where his light doesn't reach. Wait for the second guard to go as far left and run past again, just as before. Once beyond them both, head north through the door. In this hallway, the door directly in front of you leads to a room where two women are being held. There are no guards here, so just run and head through the next door. As soon as you do, you are challenged by three men. Take them out, just as before, and then walk into the next door. Head south, and don't bother with cloak. You'll be faced by yet another Brigand. Defeat him, and go into the door he was blocking. Inside is Donpa, sleeping, and a servant watching him. Talk to her to initiate a short cutscene, then go back out.
In the next room is a guard who walks in a "U" shaped pattern. Wait for him, in Cloak, to come up the left side, and as he is turning around, dash down the left path after him, without going into his light. Pass through the door, and walk up the locked jail cell door. Use Catch to grab a Cell Key that is sitting on a barrel. When in possession of it, every locked Cell door in the fortress will open as you approach it. You can unlock the door in front of you and battle the Brigand on the other side now. When you defeat him, go down the stairs, then in that room down another set of stairs, ignoring the cell door (it just leads out.) Walk left over to the pile of leaves, and use Whirlwind to blow it away. Once they're gone, Reveal will show a button that was hidden behind it; hit the button to open the door.
This leads to a cave-like area with water flowing through. Monsters will start appearing. Head north up the right path until you see another pile of leaves, then use Whirlwind and walk through the exposed door. Head north until you see three locked Cell doors. The center one has a treasure chest containing Power Bread, while the other two are empty. From here head south and through the door. Open the Cell door in here, and use Move to slide the large box all the way south, until it falls off the edge. Stand on it and jump left. Keep going down this path, unlocking the cell door, until you come upon another pile of leaves. Whirlwind them away, then Reveal to expose the button. Push it, pass through the door, and save. Opening the Cell Door in this room, you finally see Hammet. A cutscene begins between Hammet and Ivan; the reunion isn't long before Dodonpa bursts in. Dodonpa trips a lever and sics his beast on you.
Boss Battle: Toadonpa
Use Zephyr, Granite or Flash, and start pounding away. Toadonpa is weak to fire, so have Ivan use Kite on Garet to get to summoning Meteor as fast as possible. If it starts giving you any trouble, have Isaac use Ground on it to give yourself a free turn of healing. Toadonpa really doesn't have that much HP, so after 2-3 level 4 summons he will probably fall.
After you defeat it, the carcass will land on Dodonpa, trapping him. Hammet reveals that Dodonpa was about to sabotage your battle and was a little too slow about it. While your party is reluctant to move it off of him, Hammet holds no grudge and insists that you at least try. Dodonpa is injured, so he can't run away. Garet recommends locking him in the jail cell, which is exactly what ends up happening. Right at that time, Donpa comes down and gives his son a severe tongue lashing. Just because Dodonpa is locked up, however, doesn't mean you'll be able to wander the fortress as you please; the guards won't obey old Donpa, so you have to sneak around still.
When the cutscene ends, you automatically are taken out of the cave, including an automatic Cloak to get past the entrance guards. Go back through West Lunpa cave, and another short cutscene will begin. One of Hammet's men was already trying to sneak in, and he found you. It turns out he was actually not out to rescue Hammet at all, but instead was smuggling goods into Lunpa. He takes you to Kalay in his wagon. Hammet goes in a secret entrance (the Kalay Tunnel) and you are taken to the town. Once in town, head to Hammet's Palace. In the throne room, yet another cutscene begins. Hammet instructs Ivan to take the Shaman's Rod to Hesperia, another continent (It can be seen off the shore of Angara currently, but you cannot reach it in this game.) Your reward for the hard work is that you now have access to the Kalay Tunnels.