Variag, badly damaged and afire, had suffered heavy casualties; British, French, and Italian warships sent boats with medical personnel to lend assistance.

Castle's participation in that endeavor of mercy prompted Variag's commanding officer to present his personal sword to the young ensign in gratitude.

Detached to command submarine USS Plunger (SS-2) ("a cross between a Jules Verne fantasy and a humpbacked whale [sic]," as one junior contemporary, Ensign Chester W. Nimitz, later described early submarines) on 22 February 1907, reporting for that duty on 23 February 1907 and assuming command upon her recommissioning at the New York Navy Yard in Brooklyn, New York.

Relieved on 9 July 1909 with new orders, Castle returned to the United States on 20 August 1909, reporting to battleship USS Ohio (BB-12) on 12 October 1909 to serve as her senior engineer officer.

After detaching from New Jersey, Castle traveled to Pittsburgh, Pennsylvania, to be the assistant to the inspector of material, Carbon Steel Works, reporting on 10 August 1910.

During the fighting that day and the next, Castle's conduct proved exemplary as he "exhibited courage and skill" in leading his men, "in seizing the Customs House [one of the principal objects of the landing] he encountered for many hours the heaviest and most pernicious concealed fire of the entire day 21 April 1914, but his courage and coolness under trying circumstances was marked ..." For his "distinguished conduct in battle," he received the Medal of Honor.

[1] Commissioned lieutenant commander on 15 September 1914, retroactive to 1 July 1914, Castle left Utah on 11 May 1916, reporting to the Bureau of Steam Engineering in Washington, D.C., on 13 May 1916.

After a brief tour of temporary duty with Rear Admiral William S. Sims, during which he took passage in torpedo boat USS Wilkes (TB-35), Castle served on a board that evaluated "devices and plans connected with submarine warfare."

Arriving there on 22 November 1918, he relieved Captain Kenneth G. Castleman as commanding officer of the troop transport USS Martha Washington (ID-3019).

Under Castle's command, Martha Washington conducted seven round-trip voyages to French, British, or Dutch ports, with New York City; Hampton Roads, Virginia, or Charleston, South Carolina, serving as the eastern termini.

On 4 August 1919, Martha Washington departed New York City bound for Brest, France, on the first leg of a voyage that was ultimately to take her to Constantinople in the Ottoman Empire.

In seizing the customhouse, he encountered for many hours the heaviest and most pernicious concealed fire of the entire day, but his courage and coolness under trying conditions were marked.