Learn From Other Maps
You may have trouble finding inspiration for your maps or feel that they fall short in certain areas. In that case, look at other maps. There might be something in another person's map that you wish to include in your own. You may notice that other people include interesting features such as plot twists and unique scenery to their maps. Drawing inspiration from other maps is fine as long as your map is not a near copy.
Use Balance
Players will be frustrated with maps that are too difficult. Do everything in your power to make a map fair. At the same time, maps must not be so easy that other players are not challenged. To create a good game balance, place powerful buildings along choke points. Also, have a balanced variety of units. For example, if you give one player multiple cavalry, grant the other team infantry units to counter them. However, at the same time, give the second player a decent number of archers so the first player's cavalry have something to attack.
Use Triggers Properly
Triggers add a unique aspect to every scenario map. Triggers are essentially in-game events caused by the actions of a human player. For instance, you could move a village near a cave. The game will recognize your actions and respond. Common response triggers include the displaying of text, the movement of certain soldiers and the free creation of powerful mythic units. Include triggers plentifully. However, do not make the mistake of including too many triggers; this can make the gameplay seem congested.
Test Maps
After finishing your map, test it. Units must move through the map freely. Also ensure that triggers work correctly. Often, a simple error you made may cause triggers to act unfavorably. Check that the game is winnable. If you cannot beat the map, it is unlikely that other players will as well, especially considering you know that map like the back of your hand. Obviously, if any errors in maps are found, immediately correct them.