Vicksburg

Grant and Sherman launched a couple of attacks at the fortified city on the Mississippi River on May 19-22, 1863 and after being repulsed, settled into a summer siege. Until the Confederate forces surrendered on July 4, this conflict included naval bombardment, first combat for negro troops, tunneling and black powder explosions, starvation, disease, but surprisingly few casualties.
When Vicksburg fell the Confederacy lost access to the river and its last major railway, sealing a fate that was two years and thousands of American lives later.

A suitable entrance for such hallowed ground. 

The redoubts and emplacements created prior to and during the siege put the combatants in close proximity to each other.