This sally port for the fort was located at the extreme west end of Fisher's land face, next to the Cape Fear River. It seems like an awkward place for a sally port: It's in a marshy area close to the river, where it might have been difficult to build effective defenses against attack. I understand that this point was chosen as the entry in part because it was the end point of the only road leading north to Wilmington. The gate was protected by abatis and palisades, and was overlooked by Shepherd's Battery just above it.
This photo from inside the fort shows Shepherd's Battery between the two westernmost traverses, down at the end toward the left, with the sally port below to the left:
This U.S. War Department map (by Lt. Col. Cyrus B. Comstock, 1865) shows the overall plan of Fort Fisher, with Shepherd's Battery and the sally port at upper right next to the river. (This map is oriented with the northwest toward the top.)
And here's a detail zooming in on the area of The Bloody Gate:
From this layout, it looks to me as if the Wilmington Road just comes right into the fort on pretty much a straight shot. You can see that the area is marshy, and the road seems to cross over a water obstacle on a bridge. I know that Fort Fisher was more of a coastal fortification and wasn't exactly conceived to withstand a ground siege, but I might have expected to see that road taking a more pronounced zigzag path, with a protective traverse in front of the gate running parallel to the main parapet. Maybe other members here might know more about the rationale for this design. (@NFB22 or @jrweaver?)
It could be that Maj. Gen. W.H.C. Whiting, who oversaw construction of the fort, just didn't anticipate the fierce assault that would be directed at that point on 15 January 1865.
On 13 January, Union Maj. Gen. Alfred Terry landed three divisions on the peninsula north of Fort Fisher, then moved two of his divisions closer to the fort on the 14th. On the 15th, Rear Adm. David D. Porter of the U.S. North Atlantic Blockading Squadron battered Fort Fisher and silenced most of its guns. A landing party of sailors and marines assaulted the sea face of the fort. This attack was repulsed, but soon Terry's forces (his Second Division under Brig. Gen. Adelbert Ames) began their attack on the sally port at the other end of the land face. That led to the fabled engagement at the Bloody Gate.
Ames's soldiers had to cut through the abatis and palisades under heavy fire, before storming the westernmost salient and the entry gate. Dr. James Mowris, surgeon of the 117th N.Y. Infantry, described the assault this way:
"It was an awful moment, and, while with compressed lips our troops were breathing a silent petition for home and country, the signal was given, and the line, despite the storm of bullets and cannister which strewed the interval with dead and wounded, rushed forward like a tempest, through the stockade, and up the parapet, and, in a trice, a veteran Union flag fluttered on the parapet. If the roar of artillery abated, it was more than supplied by the yelling and the din of deadly musketry." (A History of the 117th Regiment. N.Y. Volunteers. Hartford: Case, Lockwood and Co., 1866. Pages 168-9.)
Here's a map of the assault at the Bloody Gate, taken from interpretive signage at the Fort Fisher park:
This is the view from the approach to the gate likely used by Ames's men: