Search for the Sleeper
According to legend, in 1480 Harbin Zemo single-handedly confronted an invading horde with nothing but a sword. Zemo slew every single invader, and the people rewarded him by promoting him to nobility, effectively making him the first Baron Zemo. He commissioned to have a castle built in the remote mountains, though its true purpose was kept secret from all but those of his bloodline.
The Castle was constructed with the intention of discovering something ancient beneath it known as the "Sleeper", a cause which the Baron found undeniably vital. He continued to oversee upgrades to the Castle's design that were many years ahead of his time, but sadly would not live to see his quest come full circle.
The third Baron, Heller Zemo, was the one who completed Harbin's intended quest from the construction of Castle Zemo by being the one to discover the Sleeper beneath the mountains the Castle was constructed on. The massive size and advanced technology confused Baron Heller, but like his ancestors before him and those who would come after, he would not give in, and he began to study the Sleeper to find a solution.
Heller's studies allowed for multiple advancements in technology, and slowly subsequent Baron Zemos began to uncover the Sleeper, allowing the excavation to continue.
The eighth Baron, Hilliard Zemo, is remembered best by the others as a naive fool. Hilliard believed that the Zemo family should use their immense wealth, power and intelligence to educate the peasantry, a notion that clearly did not catch on in his family line.
Upon his deathbed, the eleventh Baron, Herman Zemo, detailed the Zemo family history to his son Heinrich. Learning this history filled Heinrich with such joy and pride that it almost eclipsed the sorrow of his father's passing.
Heinrich was by far the most arrogant of the Barons, claiming that he was the greatest of them and that it would be he who would awaken the Sleeper. He mused in his journals, chronicling the quest the Zemo Bloodline had long taken, about the Sleeper's origins as possibly hailing from aliens or gods, as well as thinking that "Emperor Zemo" had a nice ring to it.
During this period, Zemo was contacted by a man named Johann Schmidt who called himself the Red Skull. While Heinrich confessed that it was an overly theatrical title, the Baron decided he would meet with him. The two held their discussion in the ballroom of Castle Zemo, though the Baron was disappointed to find that the greatness was lost on Schmidt. He noted that the Red Skull had a single-minded drive for the acquisition of power, and agreed to further meetings, believing an alliance would be mutually beneficial. Over time, the Baron came to admire the way Skull manipulated the political trouble in Berlin to create an army loyal to him: HYDRA, and he agreed to let the Skull post troops in the Castle as they worked.
Zemo gained a dislike for Arnim Zola during his partnership with the Red Skull believing that while he was certainly a genius, it did not make up for his odious personality and he considered the Swiss scientist reckless over ambitious. Zola's work in Castle Zemo made Heinrich nervous, and he eventually confronted the scientist over the danger his work served to both his home and the treasure contained within. Baron Zemo took his complaints to Red Skull, who forced Zola to back down and restricted him to work only on research. When they arrived, Zemo found he was disgusted by Madame Hydra, who he thought displayed too much "mannish affections" and wore too much "scandalous attire", and Baron Wolfgang von Strucker, who he thought was pitiful for wearing the title Baron yet allowing himself to be ordered around by the Red Skull and Zola "like a common domestic".
With his research suspended, Zola entered the Zemo Family library and indulged himself in the history of the Zemo bloodline, initially something Heinrich approved of, believing Zola would learn some real character from it. Whatever Arnim Zola discovered in the libraries, he soon brought to the Skull, and the two began to constantly seek for Heinrich to share more of his family history with them. In short order, this increased interest warmed Zemo up to the two again, and he consented to allowing Zola to begin experimenting in the Castle again, under his supervision. Arnim Zola claimed that he was willing to help him improve Adhesive X, a pet project of Zemo's and a powerful bonding agent, but when they performed an experiment on it, Zola rigged it to cause an accident meant to kill Zemo. Against all odds, the Baron survived the accident, though not without scarring and near-fatal damage. Like his new allies, Zemo continued to pretend it was an accident, while he slowly began to plot his revenge.
Unfortunately for the Baron, during the time he recovered the Red Skull went about destroying the soldiers that had remained loyal to him, and converting the weaker minded ones to his own cause. He found himself a stranger in his own home, and was dismayed to admit the balance of power had shifted, and HYDRA was in control now. It became clear to Heinrich that the Red Skull knew of the Sleeper all along, and he was only still alive to give him information. He decided that if he was to die, he would do so like a true Zemo, and prepared to take vengeance to defend his home and the treasures guarded by his family for generations. He watched on in rage as HYDRA fully occupied the castle, and began to shape it in their own image, but he remained the illusion of being invalid, waiting for his chance.
In 1944, Captain America dropped down in a village near the castle and disabled anti-air forces so that the Invaders could enter the village. He then proceeded towards the armory in order to destroy the weapons before they could be sold to the Nazis.
Having reported to Peggy Carter that the Invaders were clear to drop, James Montgomery Falsworth questioned whether all the guns were destroyed just as a radio dish and a hidden cannon emerged which Captain America identified as more of Arnim Zola's dangerous machines. Unable to reach the cannon in time, he headed to the building housing the radio dish, encountering more and more of HYDRA led by Baron Wolfgang von Strucker. Captain America planted charges along the steel support beams of the radio dish tower to destroy it when Strucker intercepted him, engaging in a fist fight. The radio dish was destroyed, but the cannon still hit one of the wings of the plane carrying the Invaders. Fortunately, they all parachuted out in time. Strucker surprised Captain America and managed to knock him unconscious as they both fell from the exploding tower.
Captain America was then taken to Zola's lab by Iron Cross and saw Madame Hydra give orders for an analysis of his shield. After a brief talk of how his blood was taken, Zola showed his new project involving more test subjects containing the Super Soldier Serum from the Captain's blood. Zola "allowed" the Captain to live only to study his skills. He shortly broke free and proceeded through the base to destroy the samples of Zola's experiments for an army of Super Soldiers, dispatching HYDRA troops and making his way to a radio to contact Peggy. He learned that some of his friends were being held near his present location. Captain America recovered his gear and proceeded to free Bucky Barnes, who informed him of other P.O.W.s in the base, giving Captain America a radio communicator to keep in touch while Bucky got them to a train. As he made his way to rescue Dum Dum Dugan and Falsworth, Zola's experimental Super Soldiers escaped.
Captain America managed to find Dugan strapped to an examination bed in the middle of a brawl with HYDRA forces. After freeing him, he pursued Madame Hydra who had taken Falsworth. He caught up to her and in the exchange, Madame Hydra's face was burned when Captain America threw the shield at a munitions pile. She fled, but he did not follow, instead pursuing the Red Skull, having just arrived with the Tesseract, intending to use it to activate the Sleeper. Captain America used a train to enter Zola's special lab where he was examining the Sleeper. Red Skull arrived wanting the upgraded Super Soldier Serum Zola synthesized from the Captain's blood. Captain America followed and destroyed the sample, enraging Red Skull who ordered Iron Cross to kill him. Captain America defeated Iron Cross and pursued Skull, only for the Sleeper to awake and crumble the lab. Zola escaped and activated a robot body for a later use. Captain America awoke and found a map of his location, using it to locate Falsworth.
Having finally located Falsworth, Zola's robot arrived and attempted to kill Captain America. After defeating the Zola Bot, he freed Falsworth as Zola's voice was heard taunting them. Falsworth revealed that something big was underground that not even the planes could stop. Captain America stated that he would go down below to stop the weapon while the others tried to stop it with mortars and air strikes. He also stated that when the evacuation plane came, they should meet up with it, with or without him.
Captain America made his way to the courtyard to confront the Sleeper which shot down one of the airplanes. With Dugan's help, he managed to destroy the Sleeper. Upon escaping the exploding castle, Captain America managed to rendezvous with Bucky, Dugan, and Falsworth.
During his time in the Castle, Captain America discovered Heinrich Zemo's diaries, allowing him to put together the pieces of the underlying mysteries of Castle Zemo.