TARDIS exterior staircase

The TARDIS staircase was a vertical pathway between the planetary surface and the TARDIS when hovering in the sky. The entire structure, when viewed from below, was either invisible or cloaked by a perception filter. (TV: The Snowmen)

The lowest few metres were traversed via a retractable ladder extending down from a landing, much like a fire escape ladder. Although invisible from the ground, the ladder could be pulled down by someone who knew precisely where it was and could reach it. (TV: The Snowmen)

The majority of the structure, between the landing and the cloud-top TARDIS, consisted of a spiral staircase. Like the TARDIS itself, the staircase was dimensionally transcendental; a comparatively short climb would traverse considerable altitude. Moreover, the spiral's twist reversed just below its top. For most of the staircase's length, the user climbed up clockwise and down anticlockwise; the top few steps, however, spiralled up anticlockwise and down clockwise. (TV: The Snowmen)

History
In the wake of losing his companions / parents-in-law, Rory Williams and Amy Pond, the Eleventh Doctor became a reclusive nihilist, yet could not bring himself to abandon Earth and humanity entirely. He 'parked' his TARDIS in or before 1892, atop a cloud, high above London. The staircase provided the Doctor a means of inconspicuous entry access to and egress from the surface, while assuring him solitude. A few meters of super-dense water vapour formed a footbridge between the TARDIS' doors and the staircase. The staircase's ladder terminated in a small park in an affluent neighbourhood, a short walk from the home of the Doctor's friend Vastra, her wife Jenny and their employee, Strax. (TV: The Angels Take Manhattan, Vastra Investigates, The Snowmen)

On the evening of 23 December 1892, a curious Clara Oswald followed the Doctor and witnessed him pull a ladder down from nowhere before climbing up and disappearing. Minutes later, Clara managed to grab the ladder and pull it to the ground. She knocked on the TARDIS but hid behind it out of site of the Doctor, before descending back to the surface. (TV: The Snowmen)

The next day, Christmas Eve 1892, Vastra alerted the Doctor to the strange goings on at the home where Clara was posted as the governess. The Doctor navigated his cloud – and with it the TARDIS and staircase – to the manor house, parking it above the roof. Doing so later facilitated his and Clara's escape, and lured the Ice Governess away from the children and others present in the house. (TV: The Snowmen)