Since I got my iPhone back in December I've been downloading games for it and trying them out. Some of them have been good and some terrible. But I've found one of them to be extremely addicting, especially now during my trip to Spain. Its name is Fantasy Defense, a tower defense game where you place units around a map to attack monsters and stop them from reaching a gate. Very simple, yet, incredibly fun and pretty.
In each map, monsters of many kinds will travel around a set line to try and reach a gate you need to protect, to do so, you slide your finger around the screen positioning Knights, Archers and Sorceresses around to deal damage and kill them.
Each unit has its own properties, there are Knights who deal huge damage but only have melee range, Archers who have the best range but deal the least damage and Mages who are somewhere in between, decent damage and range. Each kind of unit can do different things as well. Example, there are knights with hammers who stun enemies to stop them for second, archers who deal "damage over time" with their arrows and mages who freeze enemies to slow them down a little.
There are many ways to level-up and upgrades to buy and it works wonders to pick it up for a few minutes and have fun in either a bus, waiting room or something like that. I truly recommend the game. I have no idea if it's available for other platforms other than iOS but it seems to be free right now, so don't miss it!