It's not that the spell system itself resembles MM6, because the promotion system is entirely different (so you may learn different spells at different levels). The only specific similarity I mentioned was that MM9 had no invisibility spell and neither did MM6.What was similar about the two games, IMO, was the plot framework. And the fact that you can access dungeons way too hard for your party (e.g. Verhoffin's Ruins) at almost the beginning of the game (along with dungeons suitable for your party level).
