The Marin Headlands provide a great place to enjoy some lovely coastal scenery.  They were also a militarily significant location for many years.  Protection of the entrance to San Francisco Bay meant that a string of forts were built along the coast here.  The high ground provided an excellent base for large guns to take on an attacking fleet.  Some of these forts were operational while other emplacements were built just in time to be considered obsolete and they were never finished.  Now you can walk through the gun emplacements and imagine what they would have been like when operational.