If you have the time and resources an army of 150 at an unexpected time can be fatal to a civilization. While the enemy if focusing on economy deep into the imperial age like most do, build a small defense and then a massive offensive army. for example 150 Persian elephants (my favorite). Just when they aren't expecting it, plow into their wall. They will always put up some defense but it's never enough. When you get through and into the town spread quickly. This gives them no chance to run to the back or sides. Focus on destroying houses. Without houses they can't build an opposition force. Then work on castles, then town centers.This also cripples their population. Then in a last strike destroy the markets for a cripple to economy, and then military buildings. While this is going on feel free to slaughter villagers, but never chase them. That always loses your momentum in the destruction of their town. If it doesn't seem good enough it is always helpful to have another army being produced while this is going on. Send it in right as your last efforts are dying off.