The Iberian settlement of Arse-durgui (or Arse d'Urgui) was on the hill of Castellciutat. At that time the hill was called "Puiolo Urgelli". Where there was the village of Arse-durgui they built the city of Urgell (in Latin "civitas Orgellia" and in Catalan "ciutat d'Urgell") and here lies the origin of the name Castellciutat. Until the eighteenth century the town was known simply as "Ciutat" (City in English) and "Castell" (Castle) was added because the existence of an ancient castle that was home of the Counts of Urgell. Shortly after the destruction of the former Orgellia new neighborhood arose, which became the new home of the Diocese of Urgell. Its name "vicus Sedes Urgelli" indicates that the town is the episcopal see of the Bishop of Urgell. Over time, "Sedes Urgelli" took importance and lost the word "vicus" (denotes that it was a district) while "civitas Orgellia" lost the word "Orgellia" and becomes simply "Civitas" until the 18th century. The city is at the confluence of two rivers, the Segre and Gran Valira, located in a plain 7 km long and 691 meters high surrounded by high mountains, such as the Cadí mountain range. The plain is called plana de la Seu and was formed by the Segre river. Because it is located at one of the major crossroads of communication in the Catalan Pyrenees, la Seu d'Urgell has had a unique and historic role in the county of Urgell. The roads to Andorra, Cerdanya and the low Segre all converge on la Seu d'Urgell. Parallel to the Valira river rises a mountain range, where stand the remains of two great fortresses, the Ciutadella and Ciutat Castle (in Castellciutat), both of historic importance in the town. Small banks or open plains around the Segre are separated by gorges. In the north, La Seu d'Urgell has borders with the municipality of les Valls de Valira, to the west with Montferrer i Castellbò where there is La Seu d'Urgell Airport (also known as Andorra-Pirineus airport), to the south-west with Ribera d'Urgellet, to the south-east with Alàs i Cerc, and to the north-east with Estamariu.