Civilization 4 marked the start of a new generation of Civ games. It was the first in the series with a 3D engine, the first built from the ground-up as a multiplayer game, and the first with accessible modding. As game designer Soren Johnson told Fraser in November, he wanted to "start from scratch and re-evaluate everything" about the series up to that point. It's still worth playing today, and Twitch Prime subscribers can now do just that for free for a limited time.
All you have to do is head over to the game's Twitch page (or click the 'Prime Loot' icon next to your user name) to get your free copy—you'll need to download the Twitch desktop app to actually play it. You can do that here.
It's the latest in a long line of games that subscribers to the service, which is included with Amazon Prime, have received for free. Devil May Cry was on offer earlier this week, and other recent free games include Superhot and SteamWorld Heist.
Alongside free games, Twitch Prime subscribers get ad-free viewing, discounts on games sold through Amazon, in-game loot (like Overwatch crates), and emotes for Twitch chat. As I've said before, I don't think it's worth the cost of entry for Amazon Prime on its own, but it definitely sweetens the deal.
To link your Amazon Prime account with Twitch Prime, go here.