The degree of work involved in creating the catacombs was staggering. Icethorne's residents weren't Gods nor were they like Frost who could utilise creation/divine energy to manipulate land and space in whatever manner they saw fit, no they had to rely on mortal means. Physical effort, tools and a great deal of magic over a period of decades perhaps even centuries to finally complete their secret, underground highway system.
"These catacombs are a result of many years of painstaking effort, a hidden highway that allows our forces to travel through the nearby mountains unseen and unheard." Casper spoke with an air of pride. Icethorne's catacombs were a great source of confidence and reassurance for the military, allowing them to engage in devilish warfare and even provide them with a means of escape in the most dire of moments.
Though they were under siege the War Ursa Battalion was not trapped.