Firaxis has done a good job of bringing their over-aggressive Ghandhi franchise to smaller screens, but from today, Civilization VI will be available on the iPhone as well.
Handled by Aspyr, the game is a universal app and thereby free for everyone who already owns Civ VI on iPad. If you don't own an iPad, or haven't bought Civ VI yet, it'll cost you $37.99 to unlock all the leaders and features.
You'll also need a pretty modern iPhone to run the game at all. The minimum requirement is an iPhone 7 or iPhone 7 Plus, but ideally you'll want an iPhone X, iPhone 8 or iPhone 8 Plus. (It doesn't mention the XS, XS Max or XR, and while the game will probably run quite nicely, it may not be optimised for the newer A12 processors or their screens, particularly the XS Max.)
The iPhone version is the same as iPad, however, which means no expansions. The DLC factions, including Australia and Poland, are available for $7.99 a piece though. But beyond that, don't expect special optimisations — you'll still get plenty of late-game lag when several civs are kicking, for instance.
But if you don't have an iPad, and want a proper strategy game on your phone? Well, it's Civ. It's hard to do much better than that. And you can also play the game for 60 turns with a random civilisation for free, although you'll have to fork out $38 beyond that point.
Head here for the App Store listing, or just search for Civ VI on your phone.