Following the release of Dungeon Geomorphs in 1976 and 1977, TSR announced the release of Outdoor Geomorphs Set One, Walled City at Gen Con X in 1977, the same Gen Con at which the new Basic Set edited by J. Eric Holmes was featured.
Just as Dungeon Geomorphs were characteristic of the maps for the megadungeon beneath Greyhawk Castle, Outdoor Geomorphs Set One, Walled City were likely modeled after the layout for the nearby City of Greyhawk in Gary Gygax's original campaign.
What is less commonly known, is that a number of additional Outdoor Geomorphs were planned, but never published. Here's a snapshot from the back of the 1st printing of the AD&D 1e Dungeon Masters Guide, released at Gen Con XII in August, 1979:
Dungeons & Dragons Playing Aids, as listed in the 1st printing of the AD&D 1e Dungeon Masters Guide (1979)
From a historical perspective, it makes sense that the Geomorphs line was abandoned. Megadungeons and city adventures, intended to be created by individual DMs (without any published examples) were being supplanted by adventure "modules".
Still, I can't help but wonder what Castle/Fortress or Ruins geomorphs might have looked like. Ironically, they would have probably been more useful than the original Dungeon Geomorphs for assembling self-contained, set-piece adventures.
Incidentally, The Acaeum lists Set Four, Rooms, Chambers & Passages as part of the Dungeon Geomorphs series, rather than Outdoor Geomorphs (which makes sense, but isn't in keeping with what's indicated, above).