Some things you need to know

From AgeofWiki

Preventing army to go into autoformation

If you create a large army or if you placed a few artillery pieces together they sometimes move automatically in formation after 5 seconds you start the scenario. A few tricks to prevent this is letting your army move a little bit by creating a army with your army editor first and then let your army move with the army move trigger.

Let the enemy counter attack once you destroyed the wall or building.

imagine: You got a AI army or you army and you let them attack an enemy outpost and you want the enemy soldiers to attack you the moment you attack the walls. You have to use the condition 'unit dead' and create several unit dead conditions to cover the whole wall and click 'OR'. So once you break through the wall the trigger activates. as trigger you have to use the 'teleport trigger' and create the enemy army you want to attack the attacking army at the edge of the map where they can't move and you won't see them. And set the source units and set the area where you want them to teleport. To prevent them just doing nothing or something else you dont want them to do once you teleported them to the spot, create a new trigger and use as condition 'distance to point'. set distance (try the create fog effect to have an idea of how big the distances are) and set operator to =<. This means that if the units are within the distance of the that point the trigger will be activated. and use the 'AI attack unit list' effect. This will let the the army you teleported attack the attacking army. You have to 'create' the armys with the army editor tho. With this you can let the 2 armys fight a big melee combat once the walls are destroyed :). Or if you make a big fort with more wall layers you can create attack waves after every wall destroyed.

These are some basic things, I hope this will enlighten you :)