you have to advance to the final age, build every building, and research everything, then finally after about an hour and a half of game play you can train the legions. (but be carefull, dont kill the monk, i would suggest saving the game as soon as you train your legions before you attach) this may not be the actual requirements programmed in the game for a trigger, but when i had the same problem i tried it and it worked. it's a royal pain. usually u end up migrating to the north east corner of the map, battling to take it over because it is right next to the opponets fort. otherwise you will run out of wood and gold.