Nurmengard is a wizarding prison. In terms of magical defences, its specific protective enchantments remain unknown, but it is known to have anti-Apparition wards (similar to Hogwarts). Non-magical defences include high walls for the fortress and it may have had a guard population, similar to Dementors once guarding Azkaban (though was implied to have since become disused and automated without the need for a human presence).