Zynga‘s Empires & Allies game has rocketed to nearly 10 million users in nine days. That’s a remarkable start for any Facebook game, but for Zynga that is becoming a predictable result.

Zynga has more than 249 million monthly active users on Facebook, according to AppData. Empires & Allies, a combat social strategy game which is akin to “CityVille meets Risk,” has more than 9.5 million monthly active users as of Friday. And it gained more than 5 million users on Friday alone. That makes the game one of the fastest growing in Facebook history.

This combat strategy game has the Zynga style, which is light-hearted, cartoonish, and casual. It is the first game made by Zynga’s Los Angeles game studio, which has a number of former Electronic Arts employees who made war strategy games such as Command & Conquer for the PC.

The blending of combat and casual is a big risk for Zynga, since many of its fans are middle-aged women who may not be a natural target audience for a game where you lob an artillery round at a good friend. But so far, it looks like the bet is paying off.

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If the game continues to get users, it could reach a much bigger audience than a hardcore game would typically get. The combat strategy element will address the tastes of hardcore gamers as well as many mainstream gamers who have complained that there isn’t enough game play in Zynga’s other games, such as FarmVille.

With this game, Zynga is pushing into the domain of hardcore game publishers such as Electronic Arts and Activision Blizzard, who have only begun to deploy hardcore game properties on Facebook. (EA has launched its Superstars series of sports games on Facebook, while Ubisoft has created games for Assassin’s Creed and CSI: Crime City).

Hardcore gamers on Facebook are a relatively untapped market, served only by a few game companies such as Kabam. But Kabam, which raised $85 million last week, has discovered that those hardcore gamers are willing to spend a lot of money in each game. That may be what drew Zynga to expand into this part of the market.

Amer Ajami, head of the Los Angeles studio for Zynga, said his team built the game as a fun combat strategy game, but also tailored it for Zynga’s broad market of mass market casual players on Facebook. This is his team’s first launch. The game isn’t a huge time sink like many combat games, since you don’t have to micro-manage your resources and combat units. Most Zynga games are played for 10 minutes at a time, and Empires & Allies is no different.

Unlike most of Zynga’s other games, this title has a storyline and more than 30 characters who help make the experience more engaging. For example, “Scarlett” shows you how to you rebuild your island paradise after the evil tyrant Raven attacks it and leaves it in ruins.

Your job in the single-player campaign is to rebuild your forces and take a total of 15 islands. You can enlist your friends to help you along the way.

When you attack, the game actually breaks into a combat scene, with your forces on one side and the enemy’s on the other. You can pick which unit will fire at an enemy target. You wait for the result, and then the enemy fires back. At some point, a winner is determined. On a strategic map, you can see how much progress you have made in the campaign.

Zynga also bills the game as the “most social” game it has ever built. You can enlist your friends and designate them as either friends or enemies. Your enemy friends can attack your island and take over buildings that you own. If you want to extract resources from buildings that you have lost to your attackers, you can do so, but it costs you extra energy points to do so. During battles, hardcore gamers may discover that there are tactics they can use to help tip the odds in their favor.

Your friends can help defend you against attacks, or you can defend yourself.

The game is free to play, like Zynga’s other games, but you can speed up your production or instantly produce army units if you pay real money for virtual currency. You can also buy power-ups that help turn the tide of battle in your favor.

As you expand your empire, you have to collect wood, oil and rare ores. Sometimes your island won’t have any, and you’ll have to trade with friends to get some. That’s one of the social mechanics in the game.

You can expand your base to include as many as five islands. As you do so, characters give you advice on how to build defenses, naval units, and air power. You can do research to unlock new weaponry, and at some point in the future you will be able to get combat advantage from the veteran status for your soldiers.

Ajami said Empires & Allies is very accessible for those who are new to this kind of game. Before you go into battle, you have the option of going into combat training. If you win a battle, you can collect some loot. And when you are invading an enemy’s base, you can leave a message taunting them. The combat is played out in an asynchronous manner, so users don’t have to be online at the same time to resolve combat.

If you defend a friend against an enemy attack, you can win honor points. If all you do is attack, you get black points.

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