Dr. Cossack's Citadel

Dr. Cossack's Citadel, also known as Cossack Castle (コサック城), is Dr. Cossack's fortress in Mega Man 4. After the defeat of all eight Robot Masters, Mega Man must go through the castle, which consists of four stages. Mega Man fights against the bosses 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 that Dr. Wily captured Kalinka to force Dr. Cossack work for him.

Other appearances
Dr. Cossack's Citadel also appeared in the Game Gear Mega Man. Although the beginning of the final stages shows the Wily Castle from Mega Man 4, the ending shows Cossack's Citadel exploding instead, being possibly a mistake from the game.

In the manga Mega Man Megamix, the citadel is the home of Dr. Cossack's family. However, during the Return of the God of Death story arc, it is destroyed by Skull Man (Clad in a black cloak and scythe, resembling a Grim Reaper) and Reggae leading an army of Skeleton Joes into the Citadel, kidnapping Dust Man, Pharaoh Man and Drill Man in the process.