It took me a while to actually find the final remaining zone (Space Zone) as it was somewhat hidden via an exit from another level, but it was soon completed and Wario’s Castle infiltrated.
And Wario was defeated with ease. All the bosses were pretty easy, actually, and very few of the levels gave much of a challenge. Quite different to Super Mario Bros 3 or Super Mario World, which this is a sort of portable analogue of.
It was great though, despite the easiness and slowdown. Quite impressive for a Game Boy title.