You can build both iOS and Android apps without paying anything (you need to pay if you want to distribute in certain stores, like Apple's App Store). In the case of iOS apps, you still need a Mac computer. In the case of Android, you can simply generate the apk from Unity and upload it to Itch.io.
Yes we are accepting builds from Unity. War Child will be working with a 3rd party publisher to launch the final game onto iOS & Android, so you do not require a developer license.
