A walk through the crypt-like bowels of Slotsholmen, known as Ruinerne under Christiansborg, offers a unique perspective on Copenhagen’s well-seasoned history. In the basement of the current palace are the ruins of Slotsholmen's original fortress – built by Bishop Absalon in 1167 – and its successor, Copenhagen Castle. Among these remnants are each building's ring walls, as well as a well, baking oven, sewerage drains and stonework from the castle's Blue Tower.
The tower is infamously remembered as the place in which Christian IV's daughter, Leonora Christina, was incarcerated for treason from 1663 to 1685. Guided tours in English run at noon on Saturdays.