Following the Columbian exploration of the New World, Seville was chosen as headquarters of the Casa de Contratacin in 1503, which was the decisive development for Seville becoming the port and gateway to the Indies.[46] Unlike other harbors, reaching the port of Seville required sailing about 80 kilometres (50 mi) up the River Guadalquivir. The choice of Seville was made in spite of the difficulties for navigation in the Guadalquivir stemming from the increasing tonnage of ships as a result of the relentless drive to make maritime transport cheaper during the late Middle Ages.[47] Nevertheless, technical suitability issues notwithstanding, the choice was still reasonable in the sense that Seville had become the largest demographic, economic and financial centre of Christian Andalusia in the late Middle Ages.[48] In addition, factors favouring the choice of Seville include the Andalusian coastline being largely under the seigneurial control of the House of Medina Sidonia, Seville enjoying an important hinterland and administrative expertise, and its inland location also providing conditions for military security and enforcement of tax control.[49]