I get the impression from an inspection report in the OR's that Fort Manhasset was a series of separated artillery positions. There should have been a series of trenches for riflemen to provide close in support for the guns. There should also have been a detached magazine or magazines. There is a report from a Union officer inspecting the abandoned Fort Griffin. Haven't run across any post-war reports about Manhasset. There were a number of heavy guns in both posts. They may have been spiked or withdrawn toward post in the interior. This seems to have been part of Magruder's plan late in the war before the sudden collapse of the CSA organization in Texas. I believe that Cotham's book on Sabine Pass has reproductions of sketches of Fort Griffin. I'm not sure off-hand if has anything on Manhasset.