Why was taking Vicksburg such a daunting task for the union?

This event refers to the times of the American Civil War, and the Siege of Vicksburg was the last military action in the Vicksburg Campaign. Even though the Confederate Army of Mississippi came close to the fortress city of Vicksburg, Mississippi, they eventually had to deal with the successful ending of the Vicksburg Campaign, which due to its geographical advantages degraded the ability of the Confederacy to maintain its war effort.