This is a brief outline of the history of Portown, including adventures that could be set in each time period. The dates are set in YBB (Years Before Basic) or YAB (Years After Basic). Some are from the Holmes Sample Dungeon, the rest are extrapolations:
Prehistory: A "pre-human city" of "doubtful history" exists in the future site of Portown. A vast layered underworld is constructed by the Ancient Builders.
~500 YBB: Portown is founded to link "caravan routes from the south to the merchant ships" traveling the Northern Sea. Catacombs beneath the town are constructed, where the former Lords and Ladies of Portown are buried in sarcophagi (see Room N).
100 YBB: Zenopus arrives, builds a tower on the hill over the sea cliffs to the west of town and near the graveyard. Green Dragon Inn established.
~75 YBB: Zenopus excavates beneath his tower in search of ancient treasures.
Adventure of the era: Zenopus hires PCs to investigate pre-human tunnels under his tower.
50 YBB: Zenopus vanishes when his tower in engulfed in green flame. "Several of his human servants" escape and relate that he was destroyed by "a powerful force he unleashed in the depths of the tower".
Adventure of the era: Investigate disappearance of Zenopus
~45 YBB: Town demolishes 'haunted' tower.
Adventure of the era: Investigate haunted tower prior to demolition
~5 YBB: Thaumaturgist arrives, builds or moves into a short tower (Room S) not far from the Zenopus ruins.
0 YBB/YAB: Present day. Lord Leomund (Level 10 Fighter) is the ruler of Portown. His daughter Lemunda has just gone missing. The Green Dragon Inn is a popular gathering place for adventurous types. Pirates and smugglers are very active on the Northern Sea. Bodies are going missing from the graveyard, possibly being transformed into ghouls (as in Room P) by cultists.
Adventures of the era: Sample Dungeon as written, including rescue Lemunda.
Investigate cult activities; possibly adapt a Call of Cthulhu scenario?
5-35 YAB: Quiet years. Old dungeon entrance sealed by Lord Leomund.
40 YAB: Lady Lemunda (now a L10+ Fighter) now ruler.
Adventure of the era: Return to the Tower of Zenopus, deal with new threat(s) in the old dungeon. Pirate & cultists have merged/transmogrified into Dagonites.
(Revised from a post on G+, thanks to Andy C for suggesting I add the "pre-history" era)
See also: Portown Rumors (which are all in relation to the "Present Day" time period)