If you click on the first world, you can then scroll through to see what the other worlds are, though you have to play through the previous world to unlock. There's ten available so for the enthusiast there seems to be a fair amount of choice and I've had a number of weapon/armour drops so you can play around with those.
I'm not that enthusiast so have taken the decision to delete it BEFORE I play it a little too much and ruin my experience - played in total for a few hours and got near the end of world two - it's requiring more time than I'm wanting to give to a mobile game.
I've downloaded Mini Metro in it's place so we'll see how that one does. Very nearly forked out for Final Fantasy Tactics but reviews imply that the game is locked to one device (!) I understand that save data may not have a cloud save but if the game can't be re-downloaded without being a fresh copy, that's another thing. Something I need to check as I think that sort of game would work well.