Repairing the Northern Ruins benefits you with very good items and the powered up Masamune.
To start off go to 600AD and go to the island where the Northern ruins are. Go to the cafe and talk to the man that looks like Taban (Lucca's Father). He'll say that he is willing to work on the Northern Ruins but he doesn't have any tools. Leave and go to 1000AD. Go to the bar and talk to the man that looks like Taban. He'll say to go to his house to get the tools. When you go to his house (I don't remember which house it is but it's in that village) talk to the woman and she'll give you the tools. Go to 600AD again and give the tools to the man he'll offer to fix some of the ruins. When you go there to check on his progress he'll say you need to get rid of the monsters. Fight all the monsters you can find and go back to his house. Pay him the 2000G neccessary and he will repair more. Go down stairs to Cyrus' grave. Cyrus will recharge the Masamune as long as Frog is in your party. After that go back to the architects house and he will fix the rest of the ruins for another 2000G. Go in the ruins and open all the normal treasure boxes and for the black boxes, do what you did with the rest.
