I'm trying to understand the most logical way of creating an iOS game that runs continuously in the background.

For example.. you have tower and enemy waves. The game has endless enemy waves even when the game exits. When you open the game again, it will retrieve the data that occurred when the app was closed.

I assume a database on a server would be the best solution. The values continuously increment on the server. The game connects to the server and retrieves the specific user's updated game data.