During this period, the Roman Empire became a tetrarchy (ruled by four emperors), and nearly collapsed under the pressure of internal disputes, economic depression, plague and revolts. Rome, the capital of the empire, became increasingly vulnerable to attacks from Germanic tribes. In 271–275 CE, Emperor Aurelian built a defensive city wall, which superseded the Servian Wall (built in early 4th century BCE). It was 20 feet (6 m) high, with walls 11 feet (3.5 m) deep. Approximately 12 miles (18 km) in length, it encircled Rome, including the settlements on the River Tiber and the Praetorian Camp to the northeast. Monuments were built into its walls. These included monumental gates, such as Porta Asinaria and Porta Praenestina (or Porta Maggiore), considered the city entrance. In 312 CE, the Roman emperor, Constantine, crossed the Tiber and, after fierce fighting, ended the tetrarchy. He became the empire’s sole ruler and introduced Christianity.
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