The postal code of the area is 820. Surrounded by city walls in length, Katsina is believed to have been founded circa 1100. In pre-Islamic times, Katsina's semi-divine ruler was known as the "Sarki", who faced a summary death-sentence if found to be ruling incompetently. From the 17th to the 18th century, Katsina was the commercial heart of Hausaland and became the largest of the seven Hausa city-states. Katsina was conquered by the Fulani during the Fulani War in 1807. In 1903, the Emir, Abubakar dan Ibrahim, accepted British rule, which continued until Nigerian independence from Britain in 1960. During sub-Saharan trade, the city of Katsina was known to be one of the most vibrant and strong commercial centres, and was believed to be the strongest with the Hausa kingdoms in terms of commerce, trade and craft. The German explorer Friedrich Hornemann reached Katsina, the first Westerner to do so, at the beginning of the 19th century. The city's history of western-style education dates back to the early 1950s, when the first middle school in northern Nigeria was established. There are now several institutions of higher learning, including two universities: Umaru Musa Yar'adua University and the private Katsina University. The city of Katsina is also home to a famous 18th-century mosque featuring the Gobarau Minaret, a tower made from mud and palm branches. According to the Köppen Climate Classification system, Katsina has a hot semi-arid climate, abbreviated "BSh" on climate maps.