Briefly, ...
Fort Morgan is on the east side of the entrance to Mobile Bay. Fort Gaines is on Dauphine Island on the west. They are both pre-war stone / masonry forts. A smaller fort, Ft. Powell, guarded the narrow entrance to Mobile Bay north of Dauphine Island. Various things (like Vicksburg, operations against Texas, the Red River Campaign) kept delaying moves to close Mobile until August 1864, when Farragut led the Gulf Squadron into Mobile Bay, past the forts and the ironclad CSS Tennessee. That's where Farragut said, "**** the torpedoes, full speed ahead." (Or " "**** the torpedoes! Four Bells! Captain Drayton, go ahead! Jouett, full speed!" or whatever he really said.)
Fort Morgan surrendered shortly after, to land forces which had cut if off. It's surrender closed Mobile, which didn't actually fall until a few days after Lee surrendered at Appomattox.