Fort Foster, having been placed under caretaker status for a couple of decades after World War I, was revisited and upgraded during World War II. The minefield remained the primary defensive weapon of Portsmouth Harbor. However, the tools required to accomplish the task had changed. The long range weapons of the harbor defenses had been eclipsed by larger, newer and more devastating weapons such as the 16 inch guns emplaced at Fort Dearbourne. Newer and more effective 6 inch guns were also constructed in pairs on both sides of the harbor. The longer range of the 16 inch guns also required a larger field of observation. Three very distinct observation towers were built along the Seacoast to enable the battery commanders a greater view of the area between Portsmouth Harbor, the Isles of Shoals and beyond . With the advent and acknowledgement of airplanes as an effective offensive tool against fixed positions, more mobile and flexible defensive systems were added to the existing forts. Mobile air defense and AMTB units comprised of searchlights, 37mm, 40mm and 90mm guns were now included in Portsmouth Harbor's protective defense system. Two 90mm guns were fixed in place in front of the now obsolete Battery Bohlen.
Fort Foster saw construction of a new mine casemate for controlling the outer line of mines, a pair of 6 inch guns with its accompanying observation tower and new temporary support buildings that included a mess hall, infirmary, post exchange, recreation building and barracks. Construction of all but the 6 inch gun battery was completed before the end of the war. As the war neared its end, the threat to the Portsmouth Naval Shipyard diminished and the guns slated for Fort Foster were shipped elsewhere for use. The incomplete Battery 205 had been slated to honor Major Edwin S. Curtis.