Major Castleman was a U.S. military officer who served in the United States Marine Corps. Castleman built a reputation as a can-do military officer who was charged with many important duties, from reconnaissance to retrieval of essential personnel. He was originally commander of the U.S. Marine unit SG-3. He was trying to track down Jack O'Neill and Samantha Carter but was seriously injured after falling several meters from a ledge. (SG1: "Solitudes")

A few weeks after that, he was in charge of briefing the teams that were about to travel through the Stargate to the new Alpha Site (SG1: "The Serpent's Lair").

Some months later, he was temporarily reassigned to command SG-1 while Colonel Jack O'Neill succumbed to the download from the Ancient's Repository of knowledge. (SG1: "The Fifth Race")

During the Goa'uld negotiations in 1999, Castleman lead the base defense force in removing all weaponry and assisted in general security of Stargate Command. (SG1: "Fair Game")

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