The only reason people can kill full gold garrisons is because there are no heros fighting them, with heros it would be impossible to attack any city. Edit:Well one Cleo would be already enough to screw every attacker.
that is acually really annoying, i dont always have the ressources to get my old gold units back after i had to put them on the wall to get free gold space for units i ACUALLY did want to put in the wall.
But isnt that what makes this game strategic? That you CANT just always use the best fitting troops? You have to make choices, and often hard choices, thats what makes this game interesting.