Shroomy
Adventurer
Given I never played any Planescape (I tried reading some but the cant hurt my brain) - I didn't see anything about a spire in the 4E manual of the planes. Might not have read close enough there.
The spire is from the Great Wheel cosmology; its not present in the 4e version of the planes. In 2e and 3e, Sigil floated atop an infinitely tall spire at the center of the Outlands.