Aargau ( , ), more formally the Canton of Aargau (; ; ; ), is one of the 26 cantons forming the Swiss Confederation. It is composed of eleven districts and its capital is Aarau. Aargau is one of the most northerly cantons of Switzerland. It is situated by the lower course of the Aare River, which is why the canton is called Aar-gau (meaning "Aare province"). It is one of the most densely populated regions of Switzerland. History Early history The area of Aargau and the surrounding areas were controlled by the Helvetians, a tribe of Celts, as far back as 200 BC. It was eventually occupied by the Romans and then by the 6th century, the Franks. The Romans built a major settlement called Vindonissa, near the present location of Brugg. Medieval Aargau The reconstructed Old High German name of Aargau is Argowe, first unambiguously attested (in the spelling Argue) in 795. The term described a territory only loosely equivalent to that of the modern canton, including the region between Aare and Reuss rivers, including Pilatus and Napf, i.e. including parts of the modern cantons of Bern (Bernese Aargau, Emmental, parts of the Bernese Oberland), Solothurn, Basel-Landschaft, Lucerne, Obwalden and Nidwalden, but not the parts of the modern canton east of the Reuss (Baden District), which were part of Zürichgau. Within the Frankish Empire (8th to 10th centuries), the area was a disputed border region between the duchies of Alamannia and Burgundy. A line of the von Wetterau (Conradines) intermittently held the countship of Aargau from 750 until about 1030, when they lost it (having in the meantime taken the name von Tegerfelden). This division became the ill-defined (and sparsely settled) outer border of the early Holy Roman Empire at its formation in the second half of the 10th century. Most of the region came under the control of the ducal house of Zähringen and the comital houses of Habsburg and Kyburg by about 1200. In the second half of the 13th century, the territory became divided between the territories claimed by the imperial cities of Bern, Lucerne and Solothurn and the Swiss canton of Unterwalden. The remaining portion, largely corresponding to the modern canton of Aargau, remained under the control of the Habsburgs until the "conquest of Aargau" by the Old Swiss Confederacy in 1415. Habsburg Castle itself, the original seat of the House of Habsburg, was