When you first arrive, go right and use Reveal on the gravestones. The chest in the middle contains a Viking Helmet. If you have a Floating Hat (possible creation from the Slyph Feather you got in Tundaria Tower) then keep that on Felix - it's not only better than the Viking Helmet but it's got important Wind resistance. The Thorn Crown is better on Piers, so just sell the Viking Helmet when you can.
As you try to go north, Feizhi from the original Golden Sun game will show up, providing you'd got to the point of rescuing Hsu from the Alpine Crossing, and transferred your data across to this game. She's looking for Issac, and gives you a Golden Ring for you to pass on to him when you next see him. Of course, you are entitled to equip this on any of your characters and never give it to him.
Once you're done, explore the town and do whatever you need to. You'll notice Briggs and some of the sea fighters near a door at the top of the town, and when you want to progress, approach them. They'll notice you, and run inside.
Inside, the room to the right has a barrel between beds, which contains a Sleep Bomb if you want it. Keep heading upstairs - there's only really one path you can follow, although on the third floor, head out the bottom exit to a ledge outside, and check the jar on the right for a Lucky Medal.
Keep heading upstairs and follow the path until you reach a large room where Briggs has run to. When Obaba (the grandma) gives you the opportunity to leave, say yes for the mean time. Save, and standby all your Djinn (make sure Coal is on Jenna) except Flower, Spark, Reflux, Blitz, and Shade, then approach Obaba and Briggs again. This time, refuse to leave, and you'll have to fight a giant fiery lizard.
Boss Battle: Avimander
The first turn should go Judgement, Ulysses, Thor, Ulysseys. Next, Ragnarok, unleash Reflux (on either Jenna or Sheba), Shine Plasma, Glacier. You can continue in this manner, repeatedly building up banks of Djinn, then summoning, until the Avimander goes down. Be wary of Heat Stun however, and the fact that it can do quite a bit of damage, so keep everyone well-healed.
Once you've beaten down the Avimander, Obaba will hand Briggs over to you, but Kraden will mention the misunderstanding. After the scene, talk to Obaba. You'll hand her the pieces of the trident that you've got so far - it's the legendary trident of Ankohl, and you'll need it rebuilding if you want to get into Lemuria. Anyway, the last piece of the trident is near here, so leave the cave and head to the exit of the town (stop by the Inn/Sanctum/all-purpose shop if you need to).
When you try to leave to the world map, you'll trigger a scene with Alex. He's appeared again, and he's bought some friends with him too. Karst will show up, and soon after, a person you haven't met yet. He's Agatio, and together, they serve exactly the same purpose as Saturos and Menardi did in the original Golden Sun. Anyway, you'll get threatened into picking up the pace with getting to Jupiter Lighthouse, but it won't last - you've still got to go to Lemuria and fulfill another prophecy before you even get anywhere near Jupiter Lighthouse.
When you're finished speaking to them, leave to the world map. Get onto your boat and sail along the coast to the east to the next beach. Get off, and enter the tower - Ankohl Ruins.