Complex sentence: St. Philip's Castle was a fortress guarding the entrance to the port of Mahón on the island of Minorca, rebuilt by British forces in the 1760s, around an earlier Spanish fort, the "Castell de San Felipe", which had been established about 1554.
Split into simpler sentences: St. Philip's Castle was a fortress guarding the entrance to the port of Mahón. It was rebuilt by British forces in the 1760s, around an earlier Spanish fort, the "Castell de San Felipe", which had been established about 1554.