The basic idea is about the same as Magic Mirror Deluxe II: Hit the bonus and retrigger as much as possible to unlock the vast potential, as you get an additional expanding symbol each time you land 3 or more scatters. Along the way you have a very volatile base game, that can be pretty dry or might grant you some nice hits, in the same manner as a traditional book game.
The RTP in the base game can vary a lot, from around 25-30% to actually maintaining the balance over long stretches. I wouldn't call Rise of Merlin the perfect slot for wagering though...
There are a couple of important differences when you compare to Magic Mirror Deluxe II. Apart from the top symbol, Merlin himself, payouts for full lines are only 80% of the usual awards: A full line of kings pays only 12x instead of the traditional 15x, a full line of the red dragon (2nd best symbol) pays only 160x instead of 200x and so on.
The number of free spins are reduced in the same manner: You get 8 initial spins instead of 10, and this applies to the retriggers as well. On the sunny side, Rise of Merlin feels a lot easier to bonus than its obvious inspiration, Magic Mirror Deluxe II, at least that's my impression. RoM's overall RTP of 96,58% could indicate this (MMD II has 95,63% in comparison).
Anyway, this slot (as well as the Egyptian themed version Legacy of Dead) is a worthy addition to the Play n' GO portfolio, at least in my humble opinion.