The Pergamon Altar is a monumental construction built during the reign of King Eumenes II in the first half of the 2nd century BC on one of the terraces of the acropolis of the ancient city of Pergamon in Asia Minor.
The structure is 35.64 meters wide and 33.4 meters deep; the front stairway alone is almost 20 meters wide. The base is decorated with a frieze in high relief showing the battle between the Giants and the Olympian gods known as the Gigantomachy. There is a second, smaller and less well-preserved high relief frieze on the inner court walls which surround the actual fire altar on the upper level of the structure at the top of the stairs. In a set of consecutive scenes, it depicts events from the life of Telephus, legendary founder of the city of Pergamon and son of the hero Heracles and Auge, one of Tegean king Aleus’s daughters.
In 1878, the German engineer Carl Humann began official excavations on the acropolis of Pergamon, an effort that lasted until 1886. The excavation was undertaken in order to rescue the altar friezes and expose the foundation of the edifice. Later, other ancient structures on the acropolis were brought to light. Upon negotiating with the Turkish government (a participant in the excavation), it was agreed that all frieze fragments found at the time would become the property of the Berlin museums.
Reference : wikipedia