- "Let the Sleeper rise!"
- ―Arnim Zola
The Sleeper was a gigantic robot of unknown origin. It was buried beneath Castle Zemo for hundreds of years.
The Sleeper was discovered during the Middle Ages by the third Baron Zemo, Heller Zemo. Though they could not understand what the Sleeper was, Zemo and his men realized it was something important. Castle Zemo was soon built around the Sleeper's resting place.
Hundreds of years later, during World War II, the Castle was owned by Baron Heinrich Zemo, who thought that he was destined to awake the Sleeper and use it to help him in his world conquest. However, he unwisely made an alliance with Johann Schmidt, commander of the Nazi research division HYDRA. On Schmidt's orders, the soldiers of HYDRA occupied Castle Zemo, where Schmidt's top scientist Arnim Zola studied the Sleeper.
In 1944, the castle was infiltrated by Captain America, the only successful Super Soldier from the Allied Forces. He found the Sleeper in the catacombs beneath the castle, and Iron Cross attacked him to prevent him from getting to Schmidt, who woke the Sleeper with the power of the Tesseract. Once he had defeated Gruler, Rogers used him as a ramp to leap towards Schmidt, but he was knocked the the bottom of the chasm by the an attack from the Sleeper.
Rogers recovered from his fall and alerted Jim Morita of a massive target. He made his way to the courtyard, where the Sleeper rose through the ground and fired lasers at the Allied planes, destroying one of them. Rogers defeated one of the Zola Bots and managed to distract the Sleeper long enough for him to pass the coordinates of its location to the planes, allowing them to bomb it from the air and destroy one of its eyes. Rogers defeated another Zola Bot and caught the Sleeper's attention again, destroying its other eye by deflecting its laser at it with his shield. Rogers destroyed the final Zola Bot and had Dugan shoot at the now-blinded Sleeper to expose its neck. Rogers threw his shield at the three cables within, destroying the behemoth once and for all and bringing the castle down with it.
- In the comics, the Sleeper was a robot built by the Nazis on Adolf Hitler's orders. It was commandeered by the Red Skull after World War II who wanted to use it in his conquest of the world.