Vedelem: The Golden Horde is a wave survival real-time strategy game set during the time of the Mongol invasion of Eastern Europe. As the Golden Horde bears down on you, your job is to build economic buildings in order to expand your territory, gather resources, and pump out more troops for large-scale engagements with the enemy's ever-growing numbers. All the while, you want to fortify and upgrade your lands, giving you manned walls and buildings between your central keep and the enemy army.
Vedelem is also totally free. It was made by Castle Roaches, a student game development group at the Breda University of Applied Sciences, a Dutch university with a game design program.
To survive, you'll need thousands of troops in the field to counter the thousands more your enemies bring. There are three modes: Skirmish, Challenge, and Endless, each with their own twists. It's a simple enough game, with a handful of resources to gather and three different troop types to recruit, but don't let that discourage you from giving it a try. It's pretty well-balanced for a student project, and has a lot more content than you'd expect.
You can find Vedelem: The Golden Horde on Steam.