<|im_start|>user
What game was the animated series Tank Knights Fortress based on?

Context:
Fortress is a shooter video game developed by CCR Inc and published by Netsgo. The game spawned an animated series, Tank Knights Fortress, and a sequel, Fortress 2. On November 18, 2011, service for the game was terminated in South Korea and Japan.<|im_end|>
<|im_start|>assistant
Fortress.<|im_end|>
