First off, make sure your level has no errors when saving it. The most common errors include missing or wrong done triggers and header status (number of players and script done wrong).

 

1- Make sure that the number of tribes you place in the level match the with header tribes.

This means that if you want a level with blue, red and yellow, you must not put anything from the green tribe on the level, and the header number of players must be set to 3.

 

2- Check all stone heads and triggers. Sometimes, triggers are not placed on the stonehead, causing it not to work properly. Other than that, make sure to link all the triggers with the respective effects, discoveries, units, buildings, anything that you want, etc.. Finally, make sure all the triggers options are done right - some values can make the stone head not work, so unless you know how to set those values, dont make triggers/stoneheads.

 

3- In the EDIT - HEADER - GENERAL, make sure to write the level name. You can write anything at all. Make sure to use scripts on the tribes that will be playing: if the level has blue red and yellow, the script for green can be 0. Red and Yellow's scripts should be set to 122 (if you want to upload and play the level online, or if you want to test it offline, wanting the enemy not to move or build anything). For offline tests, you can also use a Bullfrog's script (red -53 , yellow - 54 ), or other. If you know how to create your own script, make it and set the number.

 

4- Make sure to save in the right place.

Save in your Populous directory. In my case, it would be File: Save as...: "C:\Program Files (x86)\Bullfrog\Populous\levels", and then pick one of the 25 levels. I usually like to save in the level 1 for testing purposes, using the 122 script. 

 

Now you are ready to test your level. If you followed this guide, there should be no problems (but not all errors are listed here). Open Populous and play the level 1 (or the level you saved on).

In case you want to test it online, just login and upload it HERE. Then, refresh the levels in the matchmaker, and go to test levels. Yours should be on the top. You still should test it offline first, though.