To control and monitor the flow of traffic through the palace gates, the prefects used a complex passport system involving wooden and metal tallies. During an emergency, the tallies were collected and no-one was allowed to enter unless they breached the gates by force. The guards were conscripted peasants who served for a year's term as soldiers and were invited to attend a celebratory feast hosted by the emperor before demobilization. The Minister Coachman ("Taipu" 太僕), also known as the Grand Coachman, was responsible for the maintenance of imperial stables, horses, carriages and coachhouses for the emperor and his palace attendants, and for the supply of horses for the armed forces. His latter duty entailed the supervising of large breeding grounds of frontier pastures, tended by tens of thousands of government slaves. By the reign of Emperor Wu of Han (r. 141–87 BC) these contained 300,000 warhorses intended for use in campaigns against the nomadic Xiongnu Confederation. Some of the Minister Coachman's subordinates managed stables outside the capital city. These stables housed Ferghana horses that were imported or gathered as tribute from Central Asian countries. In Eastern Han—possibly due to the Coachman's influence over the transport of arms—a prefect in charge of manufacturing bows, crossbows, swords, and armor for the military was transferred from the Minister Steward's ministry to that of the Minister Coachman. The Minister of Justice ("Tingwei" 廷尉), also known as the Commandant of Justice, and was known as the Grand Judge ("Dali" 大理) between 144 BC and 137 BC and again between 1 BC and c. 25 AD. He was the chief official in charge of upholding, administering, and interpreting the law. Only the emperor, in his role as judge, was superior to this minister. The Minister of Justice was the supreme civil-appointed judge for cases deferred to the capital from provincial lawsuits. His judicial powers, however, were similar to those of the Chancellor. He could recommend changes to the law code and the granting of general amnesties to those charged with crimes. His ministry was responsible for maintaining the Imperial Prison, where trials were conducted, and carrying out executions. It is unknown whether he oversaw all of the twenty-six prisons in Western Han Chang'an, which were built to house convicted ex-officials. However, during Eastern Han, the Imperial Prison in Luoyang was the only prison managed by the Minister of Justice.