Destroyers can stil be destroyed like any other unit, but orc succumb to magic damage and spells the worst. I have also noticed there are tons of pussies in the ranks that I have played. Most people don't want to attack UD bases because of "blight disadvantage" or one measly cold tower. Most UDs I have rushed, I have easily killed a few ghouls and sometimes thier hero without fear of towers. Its only when you get deep in or the guy has necros on his graveyard is when things get problematic.