Challenging, but not frustrating. No ads! One of the best features of this game is that once you fully explored a place, you can't come back there, so you are not wasting your time with there by trying fruitless things. Also, tapping a new place for which you don't have an inventory item yet tells you so, so that again you don't have to waste your time. That leaves trying your inventory things at accessible places and that usually makes a lot of sense, so if you have a good memory...