First off, create an army.
Then, for the first one, find the effect, unit garrisoned in.
Then, look for the condition army deploy, have that, and your army, so once a unit is garrisoned in the palace, they will spawn whatever unit you want. Be sure to loop the trigger though.
If you want to know more, open up the Chariot of Fire Map, and look how he did it.
One other thing, when you post something like this, be sure to put it in the Question and Answer thread, instead of making a new topic! Hope I helped.