The South Bastion was part of the fortifications of Gibraltar, protecting the western base of the Charles V Wall. It was originally built by Spanish military engineers, later improved by the British. The South Bastion stands at the south end of the Line Wall Curtain which defends the town from attack from the Bay of Gibraltar. Another curtain wall runs east from the bastion to the base of a precipice. This wall is pierced by the Southport Gates, guarded by the South Bastion and the Flat Bastion on either side.