The fort was built in the shape of a pentagon, with five bastions. The bastions being of unequal size. The curtains were not all of equal length, and varied from 188 to 272 feet. The distance between the bastions, measured from 416 to 476 feet. The ramparts enclosed a full two acres of ground.

Fort Pitt was protected by an astonishing system of outworks. A complex of ditches, walls, ramparts, ravelins, and redoubts expanded the total area of the fort to almost 18 acres! On the landward side, a deep ditch, known as the "Isthmus", extended from the Alleghney almost to the Monongahela. This ditch, with a fortified embankment known as the "Epaulement:, provided extra protection on the landward and Monongahela sides of the fort.

Fort Pitt itself lay close to the shore of the Monongahela, leaving about five acres free for buildings on the Alleghney side, within the protection of the Epaulement and a stockade extending around the Point. This area was known as the Lower Town.

Probably due to the fact of the nearby timber being in short supply, General Stanwix directed that Fort Pitt should be a "dirt fort". The walls and bastions were built of earth. It was constructed almost precisely as the Romans built their forts, right down to the sodded slopes and brick or masonry revetments.

Fort Pitt was, in effect, a great five-sided ditch, with the earth of the ditch thrown up to form a rampart over twenty feet high and sixty feet wide. On the landward side, the ramparts were supported by strong brick retaining walls, with the tips of the bastions further reinforced by cut stone. On the less vulnerable river sides, the walls and bastions were sodded. They were covered with squares of turf laid perpendicularly to the slope of the wall, and secured with long wooden pins. On top of the ramparts, a sodded parapet 18 feet thick was erected for protection of artillery and soldiers firing small arms. Behind this parapet ran a level space 20 feet wide, providing a platform for cannon and the necessary room for recoil after firing.

A sentry walking his post on the high, windy ramparts of Fort Pitt, looked down on a kind of walled city inside the great pentagon of earth and masonry. Around the central parade could be housed from 700 to a 1,000 men. Two-storied barracks, one of brick 190 feet long quartered the officers, and two others of frame construction, provided lodgment for the men. Both the brick and the wooden ones of weatherboard and shingles, were provided with chimneys that served four rooms and furnished cooking facilities. In the brick barracks there was " a closet in each room, and other accommodations for the officers". The fort commandant's house was a fine brick building with cut-stone steps. All these long, narrow buildings could be seen grouped symmetrically around the parade, parallel to the curtain walls. But hidden away in the immensely thick ramparts were large storehouses, magazines, and casements. Most provisions and all the ammunition was stored underground. Underground was also the guardhouse and dungeons, where prisoners awaited trial in the darkness.