Austin-based Challenge Games has been around since 2006, building slick online casual games like card fantasy title Duels. It’s been been active on Facebook games for years, too, and with some success — and now it’s Zynga’s latest acquisition.

Its executives, including chief executive, founder and serial entrepreneur Andrew Busey, and its 35 employees will immediately begin serving as Zynga’s Austin studio.

On Facebook, Challenge Games has turned Duels into an app, launched Warstorm, another card-themed fantasy game, dabbled in role-playing games with office satire Ponzi, Inc and experimented in a few other areas, like sports. While its titles haven’t gotten huge, they’ve held on to some users for many months, and are generally considered to be high-quality.

The company now has 560,000 monthly active users and 140,000 daily active users; like Zynga and most other developers, it has been seeing recent declines, albeit with stronger retention for DAUs.

		Name	MAU		DAU
1.		Warstorm	560,170		64,321
2.		Ponzi, Inc.	427,803		71,883
3.		GridIron Live Football	122,525		3,270
4.		Duels	1,152		61
5.		Nobility	178		16
6.		Baseball Boss Playa Rayta	26		0

The acquisition, terms of which were not disclosed, is Zynga’s latest geographic expansion. It recently bought Beijing-based XPD Media, converting that company into a China studio. It is headquartered in San Francisco, and also has offices in Baltimore, Bangalore and Los Angeles.

Challenge Games had raised a total $14.5 million in venture funding from Sequoia Capital and Globespan Capital Partners.