At the same time Diocletian marched into Egypt against Achillaeus, whom he besieged in Alexandria, which he took after a siege of eight months, when the usurper and his chief adherents were put to death. Diocletian is said to have behaved on this occasion with unusual sternness. Several towns of Egypt, among others Busiris and Coptos, were destroyed. Constantine, the son of Constantius, who was educated at Nicomedia, accompanied the emperor in this expedition. Diocletian fixed the limits of the Empire on that side at the island of Elephantina, where he built a castle, and made peace with the neighboring tribes, called by some Nubae and by others Nabatae, to whom he gave up the strip of territory which the Romans had conquered, of seven days' march above the first cataract, on condition that they should prevent the Blemmyes and Ethiopians from attacking Egypt. Maximianus in the meantime was engaged in putting down the revolt in Mauritania, which he effected with full success. For several years after this the empire enjoyed peace, and Diocletian and his colleagues were chiefly employed in framing laws and administrative regulations, and in constructing forts on the frontiers. Diocletian kept a splendid court at Nicomedia, which town he embellished with numerous structures. He, or rather Maximianus by his order, caused the magnificent Thermae at Rome to be built, the remains of which still bear Diocletian's name, and which contained, besides the baths, a library, a museum, public walks, and other establishments. In February, 303, Diocletian issued an edict against the Christians, ordering their churches to be pulled down, their sacred books to be burnt, and all Christians to be dismissed from offices civil or military, with other penalties, exclusive however of death. Various causes have been assigned for this measure. It is known that Galerius had always been hostile to the Christians, while Diocletian had openly favored them, had employed them in his armies and about his person; and Eusebius speaks of the prosperity, security, and protection which the Christians enjoyed under his reign. They had churches in most towns, and one at Nicomedia in particular under the eye of the emperor. Just before the edict was issued, Galerius had repaired to Nicomedia to induce Diocletian to proscribe the Christians. He filled the emperor's mind with reports of conspiracies and seditions. The imperial palace took fire, Constantine ("Oratio ad Coetum Sanctorum") says, from lightning, and Galerius suggested to the emperor that it was a Christian plot.