|Ruined Tower of Zenopus by Paul Fini of The Warlock's Homebrew, as posted here|
What level was Zenopus in the Basic rulebook Sample Dungeon? In the Background, Holmes variously refers to Zenopus as a sorcerer (9th level in OD&D), magician (7th level) or wizard (11th level or higher). While it's likely Holmes didn't think about this too much when he was writing the evocative backstory for his dungeon, I enjoy this kind of thought experiment. We turn to OD&D for clues, since it was the only available higher level rules in 1977 when the Basic Set was first published.
Zenopus built a tower, but in OD&D there's no level requirement for magic-users to construct strongholds (although only Necromancers or Wizards occupy the Castles found in the Wilderness in Vol III, pg 14-15). He possibly constructed the talking bronze mask in the dungeon, which would require a Wizard ("Wizards and above may manufacture for their own use (or for sale) such items as potions, scrolls, or just about anything else magical", Vol I, pg 6, emphasis added). However, to my mind the strongest clue is simply that the second usage of wizard is capitalized in accord with a proper title: "the ruined Wizard's tower".
The use of "sorcerer" in the first sentence of the Background could refer to an earlier title ("the sorcerer Zenopus built a tower"). If so, 100 years ago Zenopus was a sorcerer (level 9) when he built the tower, and 50 years ago when it was destroyed he was a wizard (level 11 or higher). The use of "magician" in reference to excavating the cellars under his tower is then simply flavor text. Perhaps Zenopus went from 9th to 11th level while exploring the "much older city of doubtful history" under Portown that connected to his cellars.
By the way, there is evidence that Holmes actually used this dungeon in his own campaign; his 1981 FRPG book is dedicated in part to those adventurers who "plumbed the depths of the Wizard's Tower".