According to The Oxford Dictionary of Late Antiquity, the Persian ball game was an important pastime in the court of the Sasanian Empire (224–651). It was also part of the royal education for the Sasanian ruling class. Emperor Shapur II learnt to play polo at age seven in 316 AD. Middle Ages and Early Modern era Valuable for training cavalry, the game was played from Constantinople, where Emperor Theodosius II constructed a polo ground early in the 5th century, to Japan by the Middle Ages. The game also spread south to Arabia and to India and Tibet. Abbasid Baghdad had a large polo ground outside its walls, and one of the city's early 13th century gates, the Bab al Halba, was named after these nearby polo grounds. The game continued to be supported by Mongol rulers of Persia in the 13th century, as well as under the Safavid dynasty. In the 17th century, Naqsh-i Jahan Square in Isfahan was built as a polo field by King Abbas I. The game was also learnt by the neighbouring Byzantine Empire at an early date. A (stadium for playing , the Byzantine name for polo) was built by Emperor Theodosius II () inside the Great Palace of Constantinople. Emperor Basil I () excelled at it; Emperor Alexander () died from exhaustion while playing and Emperor () died from a fatal injury during a game. After the Muslim conquests to the Ayyubid and Mameluke dynasties of Egypt and the Levant, their elites favoured it above all other sports. Notable sultans such as Saladin and Baybars were known to play it and encourage it in their courts. Polo sticks were featured as one of the suits on the Mamluk precursor to modern-day playing cards. Europeans transformed the polo stick suit into the "clubs" of the "Latin" decks, as polo was little known to them at that time. The game spread to South Asia where it has had a strong presence in the northwestern areas of present-day Pakistan (including Gilgit, Chitral, Hunza and Baltistan) since at least the 15th–16th century. Qutubuddin Aibak (), originally a Turkic slave who later founded the Mamluk dynasty (1206–1290) Delhi Sultanate, was accidentally killed during a game of polo when his horse fell and he was impaled on the pommel of his saddle. Polo likely travelled via the Silk Road to China where it was popular in the Tang dynasty capital