You like SHMUPs ? (Shoot Em Ups) but you cant play them while traveling ? at least it would be a problem to take your Sega or Nintendo always with you 🙂 So why dont you use your Cellphone ? There are many SHMUPs for free to play and they are not that bad anymore. You can upgrade your Spaceship, play with friends or other plays to fight together against evil bosses. There is a Highscore ranking and reward system, so everything perfect for a near retro shmup time ? Well no the dark side : at some point you need to refresh your energy to play or wait some hours. Upgrades are harder and harder to get but an ingame store upgrades faster for real money. As long you get not addicted and spend money on such ingame items, you are fine to play for free on Android or iOs.