However the fortress proved to be impregnable. Contemporary opinion still considered the fortress impregnable. Advances in artillery made previously impregnable defences useless. However, Pemberton had the advantage of terrain and fortifications that made his defense nearly impregnable. The castle was surrounded by a triple ring of forts making its position seemingly impregnable. Yechury, along with Prakash Karat, was instrumental in creating an impregnable leftist bastion at JNU. The Tibesti then played the role of an impregnable mountain stronghold for the newcomers. The fort remained impregnable to direct assault. Its exterior walls ranged between approximately thick, making it virtually impregnable to enemy naval attack. This would make causeways impregnable against air strikes and artillery. The uncontested surrender of the supposedly impregnable fort caused a public and political uproar. Western Europe perfects castles that are impregnable. The Swedish conquest of Kronborg in 1658 demonstrated that the castle was far from impregnable. At one point, five stone walls ringed it, making the city practically impregnable. In 1226, Iltutmish captured the Ranthambore Fort, which was reputed to be impregnable.