Northwest corner of the park around 108th Street
The Blockhouse is the one remaining fortification of the four that were located in this area and it's speedy construction by volunteers is evidenced by the unusually rough stone work. The Blockhouse had a timber floor to support heavy canons and all four sides have two small gunports.
The upper two feet of the walls are different in composition from the lower walls and were probably added after the war when the building was used for storage. The stair and entrance are also not original. This rugged area was not considered suitable for park land and, therefore, not included in the original 1858 plan for Central Park, but was added in 1863 since the hills and swamp (now Harlem Mere) also proved unsuitable for commercial development. The remnants of the blockhouse were landscaped and treated as a picturesque ruin - a role that it continues to nobly serve. (reference)