The Aqua Appia flowed for 16.4 km, entered the city of Rome from the east, and emptied into the Forum Boarium, near the Porta Trigemina and the Tiber River. Nearly all of its length before entering the city was underground, which was necessary because of the relative heights of its source and destination, and also afforded it protection from attackers during the Samnite Wars that were underway during its construction.
After entering the hilly area of Rome, the aqueduct alternated tunnels through the Caelian and Aventine hills with an elevated section in between. A detailed modern model of ancient Rome shows the aqueduct running along the top of the Servian Wall above the Porta Capena. It dropped only 10 m over its entire length, making it a remarkable engineering achievement for its day.