What game was the animated series Tank Knights Fortress based on?
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.
Fortress.
Below is an instruction that describes a task, paired with an input that provides further context. Write a response that appropriately completes the request.
### Instruction: What game was the animated series Tank Knights Fortress based on?
### Input: 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.
### Response: Fortress.