The Iberian kingdoms on Aerth seem to have the same names as some of the real-world Iberian kingdoms, though, there is no Reconquista. (Also, Atlantis is real, and at least one of those kingdoms follow the Atlantaean pantheon).
With regards to the Reconquista, some kind of a strategic level wargame would be interesting to see (say, like Axis or Allies, or Risk), something that would advance the timeline over that 700 year period of history.
Say, 10 year turns.
For a 2-player game, each would control a single kingdom, but, during the course of the game, would be able to ally with other kingdoms, and thus form a bloc of allied kingdoms.
Each turn, I could see several options for the player:
* Trade
* Diplomacy
* Warfare
The tricky part would be how to 'update' the map, with the rise and fall of various kingdoms. Maybe little flag-tokens could represent the various areas of control.
With regards to tactical-level wargaming during the Reconquista, the BATTLESYSTEM rules are fine for that.
As a final note, the best rules for this kind of a campaign are not to be found in AD&D, but in D&D (see the Rules Compendium). D&D provides much better rules for strategic level events.
(Although some might say Birthright is the best, but I'm not familiar with these rules).