As the son of Mars, and as founder and first king of Rome, Romulus is synonymous with beginnings. Without Mars, however, Romulus is nothing, so it is through the concomitance of Mars and Romulus that the origin of Rome takes place in Piranesi's Campo Marzio.
The presence of Romulus within the Ichnographia occurs both directly and indirectly. There is the Templum Romuli, and there is the Templum novum Quirini. The Templum Romuli is within the Campus Martius proper, while the Templum novum Quirini is situated further east between the Horti Salustiani and the base of the Quirinal Hill. Both temples, interestingly enough, "preside" over large stadium complexes, and it is worth further noting that the Templum Romuli is on the same axis as one of the Ichnographia's two Templum Martis.
The third and most indirect reference to Romulus is the Lineae indicantes viam triumphalem--the line indicating the Triumphal Way--which begins in the Area Martis in front of the Ichnographia's other Templum Martis that is on the west bank of the Tiber within the Vatican valley. Each Roman Triumph, of which there were hundreds throughout ancient Rome's history, is a reenactment of the original triumphal march of Romulus. The Lineae. . .triumphalem of the Ichnographia therefore literally marks a lineage back to Rome's inception, and thus locates a point of beginning for the Ichnographia Campus Martius as well.
Pagan - Christian - Triumphal Way