Instructions
Select a civilization that has combat bonuses, like Mongolia, Germany or Aztec. Aggressive, military-oriented civilizations have specific bonuses and unique units that aid drastically when waging war on other civilizations.
Focus your cities on producing science so you can research up to the Feudalism tech to get Knights. Make sure to have plenty of archers to defend the cities you conquer, and warrior-type units to roll over enemy cities. By the time you have Knights, you should be technologically superior to other Civs, allowing you to defeat whatever units they can field.
Use the "Save" function to save as many times as possible in order not to pass the 1,000 AD deadline. When the game starts, save immediately, then use your Warrior and Scout to find the nearest enemy city to attack. When you find this city, memorize its location, then reload the save. Set your city to start producing warriors. Send the warrior directly to the city for which you've memorized the location. Save after you've found and destroyed one city, then repeat the process over and over to locate and destroy all of the other enemy cities.