Magus' Castle is easily one of the game's biggest highlights though you're not in the part where the plot really starts moving along.

Frog is a wonderful character but honestly there really isn't a character I don't like in this game, though I do tend to bench Marle for most of the game. I can't say much about Magus because I don't want to spoil to much but he's a much more complex character than you're led to believe.