How much free space is left on the device? A game, or any app for that reason, needs room for temporary data and such. If there is none, a crash is very probable.If full, go into Settings / General / Storage and see what you can get rid of. Purging unneeded photos, songs, videos, WhatsApp/Twitter/Facebook content also frees up space.
Also, how many apps do you have loaded at the same time? A double tap on Home will produce a list. If there are many, memory (different from storage: think about RAM and the hard drive on a PC) may be tight. Force-quit unused apps and see if that helps: from the list after the double-tap, flick each up and out. This does not erase them or cause data loss, just unloads them from the active processes list and frees up their resources.