Using the Aztec civilization is the easiest way to achieve this achievement. First thing you want to do, is go into your city screen and customize your workers so that you have them all on science production. Then you’ll want to rush a warrior unit. Once you leave the city screen, save your game. And choose to research for Horsemen.
From here use your warrior to determine where the closest city is…it should be only 5-6 turns away at most. Once you’ve discovered the closest city, reload your save and move your warrior there ASAP. Chances are you’ll get there as they produce their warrior and maybe you’ll get lucky enough that they’ll take him exploring and you’ll have a free city. But either way, save before you attack, but then attack and take over this city, if your warriors loss, then just reload the Saved game and redo it. I was able to get mine the first time.
Now you’ll want to save a again, and again go out searching for the rest of the cities. Once you’ve found them, Load up your saved game again, and us your warrior to go exploring…possibly finding gold and technologies for you. Once you’ve got horsemen researched, change both of your cities to production, and start mass producing horsemen. I was able to take over my first city with 1 Horse Army, and an extra Horse unit. Use the time in between horse production, to take a Horse unit and go fight some barbarians. This will allow you to get Veteran status and possibly an elite status, like infiltration, before you go fight your first city.
Main thing to remember is to save before you attack a city and if you’re successful then save after you take it over. Once take over cities keep mass producing Horse unit and make Armies. My third city I took over with 3 Horse Armies, and my fourth and final city took me 2 rounds with 4 Horse armies to win. I finished in 600AD and only had to restart once…and that was trying to take over the third city, I got completely beat up, so I reloaded and tried again, and ended up winning that time.
Keep in mind that defensive units are not needed, unless you see enemies coming close to a city, in which case just rush an archer, then go back to making
Wasting turns is the worst thing you can do. So if you think you could have done something a lot quicker just reload from last save point.
Us your best units first when attacking a city, this will give you the best chance of winning, while also allow your other units to have a decent shot at winning if your first unit fails.
Sometimes not joining 3 horsemen into 1 unit will give it 3 attacks instead of 1. This is useful when you need to clear some last units of warriors.
If you get a great person, don’t settle them, use them right away…esp. if they’ll give you gold.
Before you attack later cities…make sure you have all your units with you and ready to fight before you start war. If need be take a few turns to gather up your armies, and if you can bring a spy with you, it’ll only help you out.
If all the capitals aren't on the same island, start a new game, it's alot easier and time effiecient if you don't have to travel over seas.