Two hours is a comfortable visit and an hour is enough if the children are small. Prioritise in roughly this order.
The casemates. The arched brick galleries inside the walls are the single most impressive thing on the Alabama coast and photograph beautifully in low side light. They are also cool, which in July is not a minor consideration.
The parade ground and the citadel footprint. Standing in the middle gives you the geometry: a pentagon with a bastion at each point, designed so that every face of the wall could be covered by fire from another.
The concrete batteries. Grey, angular, utterly unlike the brick, and dropped straight into the nineteenth-century fort around the turn of the twentieth. Battery architecture like this exists all along the American coast and is almost never seen next to a masonry fort in the same walk.
The water. From the ramparts you look straight across the bay entrance to Dauphin Island, with the ship channel running between. Bulk carriers heading for the Port of Mobile pass close enough to read.
The museum. Smaller than the fort and worth twenty minutes, particularly for the battle material.
Take a torch. Some of the interior passages are genuinely dark, and phone lights are not adequate for the stairs.