Amazon has a new open-source, free, and royalty free game engine called [Amazon Lumberyard]. Lumberyard licensed and extends the [Crytek Engine]. Udemy offers [CryENGINE SDK Game Programming Essentials] to quickly ramp up and get started. Amazon has a [MarketPlace] but it’s still early to expect an asset store to sell game content[?] The [download page] has several game assets available.