lost on chrono trigger

ok, i just put the masamune sword together, the old guy in the portal place told me that someone can beat magus in the middle ages with it, i havent played this game in ages and just picked it up at my last save point which was the part i just described, where do i go now?

Go to 600 AD, and take the sword to Frog, he will be in a forest at the southern end of the continent, at the very back, there is a ladder behind a bush