What is the name of western comedy series about Calvary defending a fort?

You're likely thinking of "F Troop", a popular American sitcom that ran from 1965 to 1967.

The show follows the misadventures of the inept and comical soldiers of the 3rd U.S. Cavalry stationed at Fort Courage in the American West, led by the bumbling Captain Wilton Parmenter.

While not exactly focused on "defending" the fort, the series frequently features comical battles, skirmishes, and encounters with Native American tribes.