Medieval Millennium is an adaptation of Civilization II: Test of Time, based on Western Europe during the one-thousand-year period from A.D. 500 to A.D. 1500. It requires the Test of Time Patch Project (TOTPP) version 0.15.1 and takes advantage of many features that this enables.
In a technical sense, this is a complex modpack in which all units, improvements, wonders, advances, etc., have been altered to fit the medieval theme. There is no .SCN file, and every game begins on a new randomly-generated map (utilizing the new "Play a mod" menu option added via TOTPP). A game last 500 turns, at two years per turn, and you can win either by conquest or by "voyage".
On the other hand, this has many features more commonly found in a scenario, the primary one being extensive use of events to dramatically alter the gameplay experience. This allows the controlled introduction of new factors and gameplay elements at appropriate times, not directly caused by in-game actions. The experience of playing probably aligns very closely with what players have come to expect from an in-depth scenario.
In short, this is something of a unique hybrid. Perhaps the best summary is to say that this is the original base game of Civ II, completely reimagined, with the power of Lua programming behind it.
The intent is not to recreate the strictly historical sequence of events during these years, or to allow you to replay and reenact events exactly as they occurred, but rather to provide a thematically "medieval" experience or context. Every attempt has been made to provide an immersive experience that balances authenticity with enjoyable gameplay. This is far more than a graphical overlay or a new skin on familiar game mechanics — you'll encounter many detailed and nuanced changes to all aspects of the game.
Medieval Millennium 1.0
A complete redesign of Civ II covering 1000 years of Western European history