This cactus lives in a clay pot but it grows in a sub-irrigated planter (SIP aka erroneously as "self-watering") insert (note plastic rim). It was a 2" pot size plant a little over five years ago. It has spent most of its time on a windowsill here in my Brooklyn apartment and more recently on a light garden shelf. The can is a scale figure.
This cactus has been thriving in my Brooklyn apartment expect for one thing. The plant is healthy but note the brownish ring about half way up. It was caused by a gross operator error (by me). Contrary to what most people might think, it was not over-watering. Can you guess what caused it?
I forgot to water it for 3 months! It was growing on a windowsill hidden by one of my light gardens. It sort of went into winter hibernation, using little or no water from week to week. When I discovered my goofup the top of the plant had turned brown. As it grew, the brown top became a brown ring. Lesson learned! Even cactus plants need water when they need it.
See prior photos of this cactus in this Flickr photo set.