While most dads might be content to help their kids build snowmen, Utah father Brent Christensen builds ice castles.

In search of new cold-weather activities for his children, Christensen decided to build a fort made completely out of icicles, complete with tunnels, caves and a slide. The castle attracted so much attention from neighbours that the local Zermatt Resort in Midway, Utah, asked him to build one for them for the 2009 to 2010 season.

His castles have grown in scope and ambition every year since. This year, both the Mall of America in Bloomington, Minnesota, and the Steamboat Springs ski resort in Colorado have a Christensen icicle castle to call their own. The Mall of America castle covers a parking lot more than an acre wide and stands 30ft high, with 15ft-wide ice towers, tunnels and throne rooms that visitors can explore. The Steamboat Springs castle stands 40ft high, but is more compact than the one in Minnesota, with small courtyards and ice tunnels that illuminate at night with light.

Unlike other castles built from solid ice blocks, these are created only with icicles. Builders run water over custom-built racks and harvest the frozen pieces to apply them to the growing structures. About four million gallons of water were used to build the castle at Mall of America. The building teams continue adding icicles throughout the season to replenish melt-off and keep the exterior looking fresh.

Both castles are expected to stay open through mid-March, as long as the weather cooperates. Admission is $12 at the Steamboat location and $10 at the Mall of America. Visitors should wear warm clothes and winter boots to keep from slipping on the frozen floors.