In the angle formed by the projecting wall of the Prætorian camp and the Aurelian Wall, there is a gate now walled up and called simply by the name of the Porta Chiusa. This gate is one of the mysteries of Roman topography. It is not mentioned by Procopius or by the anonymous writer of Einsiedlen, yet it seems too large and important to have been altogether omitted. That a gate would be required here in Aurelian’s Wall, at least before Constantine’s reign, while the camp was still occupied, seems probable. No passage would be allowed to the public through the camp, and besides the Porta Nomentana, another gate would be wanted for the convenience of persons resorting to the camp from the country with supplies of provisions, or on business of various kinds, or for the shopkeepers who would naturally live within the walls near the camp. It may have been closed when the camp was abolished by Constantine, and that part of the city became comparatively empty, and it would thus in the time of Procopius, or the anonymous writer of Einsiedlen, have been long blocked up and forgotten or perhaps concealed by other buildings. This may account for their silence. CHAPTER VIII. THE AVENTINE AND CÆLIAN HILLS. [Sidenote: The Servian walls.] Before the end of the regal period there was an enlargement of the limits of the city in which the Aventine and Cælian were comprehended. Dionysius, Livy, and Aurelius Victor all relate that Tarquinius Priscus undertook the building of a new stone wall for the defence of the whole of the new quarters of the city, but that he did not live to finish it. The design was carried out by Servius Tullius, who also constructed the enormous agger called by his name, and still partly remaining at the back of the Esquiline, Viminal and Quirinal Hills. Before this great work was accomplished we must suppose that each suburb, as it grew out of the original settlement, was defended by a new piece of fortification, but these fortifications were, as Dionysius describes them, only temporary and hastily erected for the nonce. The expressions of Livy and Aurelius would lead us also to the conclusion that they were not of stone, but probably were entrenchments of earth. Rome then became the capital of Latium; she had lately united all her citizens, the Montani, the Collini, and the other freeholders living within the districts of Servius by a complete military organisation, and her powers were directed by a form of government which has always proved best calculated for the production of great public works. A new stone wall was accordingly planned on a vast scale, and the drainage of the low-lying parts of the city was effected about the same time by colossal sewers. The king having the whole control of the finances of the state could appropriate large sums of money for works of public utility, and could also doubtless command the labour of immense gangs of workmen. The Servian walls and the Cloacæ of Rome are to