The Sample Dungeon aka The Tower of Zenopus

The last few pages of the Holmes Basic rulebook are filled with an introductory scenario simply titled, "Sample Dungeon". This basic level dungeon replaces the "Sample Map of Underworld Level" found on pages 4-5 of The Underworld & Wilderness Adventures (volume 3 of the original D&D set). While Dr. Holmes primarily served as editor of the Basic rulebook, he authored the Sample Dungeon first as confirmed by Gary Gygax on Dragonsfoot (in response to my query), and later as shown by the Holmes Manuscript.

The "Sample Dungeon" is colloquially known as the "Tower of Zenopus". This could be interpreted as a misnomer, since the wizard's tower is long destroyed, but is actually an appropriate name in the vein of "Castle Greyhawk", which is also used as the name of the dungeons below the ruin.

Holmes' original hand drawn map of the Sample Dungeon can be seen here in a side-by-side comparison with the published version. 
Note that the header on page 40 of the rulebook that reads "Sample Floor Plan, Part of First Level", actually refers to a different partial map that Holmes included in the Manuscript but that was cut by TSR for publication. 

In the Manuscript series, Parts 46 through 54 compare Holmes' original text for the Sample Dungeon to the published version. In revising the dungeon, Gygax generally increased the strength of monster encounters while reducing the value of treasure.

Holmes' dungeon has a "circular route" layout (see Melan's dungeon layout article) that interacts well with the "outside environment" - there are at least three other "exits" from the dungeon in addition to the entrance (thaumaturgist's tower; pirate's sea cave; and the rat tunnels), a river entering the dungeon, a possible connection to ghoul-haunted tunnels under the graveyard, and a possible entrance to lower levels. Specifically, the coda of the adventure alludes to "(undiscovered) deeper levels". However, no entrances to the lower levels are shown on the map, and no clues have been found in the text.

The encounters in the Sample Dungeon contain several possible allusions to fantasy literature, as I originally noted in an article titled "Holmes Basic Easter Eggs" (this information was later incorporated into the various entries for the  Manuscript series)

DM Resources:
Sample Dungeon - official download - part of a retrospective about Basic Sets from 2008

Portown Rumors - 3-page pdf detailing 20 rumors overheard in Portown

More Portown Rumors at the Dungeons of Klang blog

Sample Dungeon Factions

A simple scenario for using the Sample Dungeon with beginners

The Tower of Zenopus by Badmike - a rewrite of the backstory

See also:
"Beneath The Ruined Tower of Zenopus" pod-play drama.
"An original scripted performance about a group of adventurers exploring the labyrinth beneath a magicians ruined tower! Dedicated to and influenced by John Eric Holmes' sample dungeon in the original blue book."

The room letters in the Sample Dungeon vertically spell out the initials of J. Eric Holmes