I'm playing Fortress Defense mod with some additions of my own.
I've got a new fortress up and running after my old one died in a war-induced tantrum spiral. I set up a military of ~15 swordsmen, 10 crossbows, and 4-5 spearmen, but I'm pressed so thin for manpower and time that I've resorted to danger-rooming new replacements up to speed as much as possible before the next siege.
I get the notice that tigermen and frogmen have assaulted the western and eastern gates. No problem, these chumps are fodder for my champions of 10+ kills each. I send soldiers to the courtyard above the central square, melee protecting crossbows a good distance ahead, then open both besieged gates. Five or six enemy squads of mixed race all charge in a massive mob but are driven back before the might of my soldiers' arms. I order my swords and spears to chase them out the east gate.
Then, AMBUSH: two squads of goblins and one of giant fiend spiders decloak right next to my soldiers, having sneaked in through the west gate. They hit my melee from behind and paralyze them while my crossbowmen struggle futilely to put a swift end to them. The last squad of mace-wielding frogs enters battle from the east, completing the encirclement of my troops. Between the constant webbing of the spiders, the armor-piercing of the frogs, and the axes of the goblins, my army perishes, and now I've barely managed to retreat the nearest survivors of my archer squad and sealed myself in while trying to train up a new squad of troops from my farmers and mason.