The northern fortress gate of the Late Antiquity fortress was placed right at the center of the wall and was flanked by two U-shaped towers. The gate was rectangular with its long side in the north – south direction. It was 10.25 m long and the width of the opening was 10.08 m (in the northern part) and 8.48 m (in the southern part). The foundations of the gate were constructed like the ones of the entire northern fortress wall: they were embedded by 1.50 m in the stable terrain and were made of limestone masonry with abundant soldiering of white mortar.
The external entrance of the gate was confined by two massive pilasters with grooves that were 0.15 m wide and 0.12 m deep. They were used for lowering of the external descending door. Behind the pilasters stood a rectangular room (propugnaculum) sized 5.78 m (north – west) by 6.18 m (east – west). Its walls were 1.95 m thick and were constructed with the opus implectum technique just like the northern fortress wall: the two faces were made of finely shaped stone quadrae soldiered with white mortar and connected with iron braces while the interior of the walls was filled with stone rubble soldiered with white mortar with admixture of crushed brick. The two walls of the propugnaculum that were 1.70 above floor level had a bay each with the width of 1.08 and depth of 0.67 – they were probably used for placing statues in. The letter A was inscribed in a specific way on stone blocks below those bays – a horizontal hasta in a broken triangular shape (only the one at the northern side of the propugnaculum was preserved). From the inside the propugnaculum was opened with a massive two-winged door that was 4.16 m wide and was placed between two pilasters. The propunaculum was probably covered by a vault arch which served as a floor of a room above. That room was used for operating the external descending door and the enemies who entered the propugnaculum were fired at through openings at the floor of the room above. After the internal two-winged door there followed a room open to the south with rectangular design that was sized 3.21 m (north-south) by 6.18 m (east-west). The letter A was hewn on stone blocks at its both sides. The letter had the same characteristic appearance as the ones in the propugnaculum. The eastern and the western walls of that room were 1.15 m thick and supported the structure of the second floor of the gate. They also confined the opening of the two wings of the internal door. A single stone staircase that adjoined the internal side of the curtain to the north of the gate led to the upper rooms of the northern gate. The stairs were constructed by stone quadrae soldered by white mortar with admixtures of broken brick. The stairs were 0.24 – 0.41 m wide and 0.24 – 0.34 m high. The foundation of the stairs was constructed not on the shoulder of the fortress wall but on a cultural layer – that is the staircase was not a part of the initial fortification system of Abritus but was constructed later. The presumed height of the staircase was 10.10 m – this was the height of a battle pathway along the upper part of the fortress walls. Fortress crenels of about 2 m rose before the battle pathway at the external side of the fortress wall. During the reign of Justinian I (527 – 565) the northern fortress wall was blocked up with a wall of large stone blocks soldered by white mortar – similar to the way the southern and the eastern fortress walls were built up.