The Harbor Entrance Control Post at Fort Stark was built in 1943 and was a joint Army-Navy installation to monitor shipping traffic and command all of the defenses of Portsmouth Harbor from Rockport MA to Kennebunkport ME. This structure was designed to look like the bridge of a massive battleship and was built on the deactivated Battery Kirk. The structure was used in the 1970s by the reserves for training.
Note: They are sealing up all the bunkers at Fort Stark this year and are considering destroying the HECP.