Tuko
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Actually, it's not entirely free. You still need to engage in battles after leveling up. I don't think I spent a long time wandering through the corridors.
Well, how is it "free"... I mean, it's about increasing stats outside of the usual leveling system.
You approach a structure on the map, and bam – you get +1 to magic or +2 to defense.
That's a freebie.
These objects are not mandatory to visit, but they are very desirable.
That's why you should visit all the islands!
And in the final battle, I didn't even engage – all the monsters were running away or being defeated automatically with almost no losses.