Pyramid of Cestius, Rome, incorporated into the Aurelian Walls near Porta San Paolo and Protestant Cemetery.

The Pyramid of Cestius (in Italian, Piramide di Caio Cestio or Piramide Cestia) is an ancient pyramid in Rome, Italy, near the Porta San Paolo and the Protestant Cemetery. It stands at a fork between two ancient roads, the Via Ostiensis and another road that ran west to the Tiber along the approximate line of the modern Via della Marmorata. Due to its incorporation into the city's fortifications, it is today one of the best-preserved ancient buildings in Rome.
The pyramid was built about 18 BC–12 BC as a tomb for Gaius Cestius, a magistrate and member of one of the four great religious corporations in Rome, the Septemviri Epulonum. It is of brick-faced concrete covered with slabs of white marble standing on a travertine foundation, measuring 100 Roman feet (29.6 m) square at the base and standing 125 Roman feet (37 m) high. In the interior is the burial chamber, a simple barrel-vaulted rectangular cavity measuring 5.95 metres long, 4.10 m wide and 4.80 m high. When it was (re)discovered in 1660, the chamber was found to be decorated with frescoes, which were recorded by Pietro Santi Bartoli, but only the scantest traces of these now remain. There was no trace left of any other contents in the tomb, which had been plundered in antiquity. The tomb had been sealed when it was built, with no exterior entrance.

The Porta San Paolo (San Paolo Gate) is one of the southern gates in the 3rd-century Aurelian Walls of Rome, Italy. The Via Ostiense Museum (museo della Via Ostiense) is housed within the gatehouse. It is in the Ostiense quarter; just to the west is the Pyramid of Cestius.
The original name of the gate was Porta Ostiensis, because it was located of the beginning of via Ostiense, the road that connected Rome and Ostia where functioned as its main gate. Via Ostiense was an important arterial road, as evidenced by the fact that upon entering the gate of the same name, the road split, with one direction leading to the famous Emporium, the great market of Rome. In 549, the Rome was under siege; the Ostrogoths of Totila entered through this gate, because of the treason of the Isaurian garrison.

The Aurelian Walls (Italian: Mura aureliane) are a line of city walls built between 271 AD and 275 AD in Rome, Italy, during the reign of the Roman Emperors Aurelian and Probus.
The walls enclosed all the seven hills of Rome plus the Campus Martius and, on the right bank of the Tiber, the Trastevere district. The river banks within the city limits appear to have been left unfortified, although they were fortified along the Campus Martius. The size of entire enclosed area is 1400 hectares.
The full circuit ran for 19 kilometres (12 mi) surrounding an area of 13.7 square kilometres (5.3 sq mi). The walls were constructed in brick-faced concrete, 3.5 metres (11 ft) thick and 8 metres (26 ft) high, with a square tower every 100 Roman feet (29.6 metres (97 ft)).
In the 4th century, remodelling doubled the height of the walls to 16 metres (52 ft). By 500 AD, the circuit possessed 383 towers, 7,020 crenellations, 18 main gates, 5 postern gates, 116 latrines, and 2,066 large external windows.
The Aurelian Walls continued as a significant military defense for the city of Rome until September 20, 1870, when the Bersaglieri of the Kingdom of Italy breached the wall near the Porta Pia and captured Rome, leading to the capture of Rome and end of the temporal power of the Pope, thus completing the unification of Italy. The walls also defined the boundary of the city of Rome up until the 19th century, with the built-up area being confined within the walled area.
The Aurelian Walls remain remarkably well-preserved today, largely the result of their constant use as Rome's primary fortification until the 19th century.

more on https://en.wikipedia.org/wiki/Pyramid_of_Cestius , https://en.wikipedia.org/wiki/Porta_San_Paolo , https://en.wikipedia.org/wiki/Aurelian_Walls