Dr. Cossack's Citadel

The Cossack Citadel is Dr. Cossack's base in Mega Man 4. This is where Mega Man fights against Mothraya, Square Machine, Cockroach Twins and Dr. Cossack inside the Cossack Catcher. After Dr. Cossack's defeat, Proto Man appears with Cossack's daughter, Kalinka, and it's revealed Dr. Wily captured his daughter to force Cossack work for him.

The Cossack Citadel also appeared in the Game Gear Mega Man and the manga Mega Man Megamix.