Few against many.

[marhcs]

Hi. is it possible that an army of 1000 troops are beeten by 19 bowmen in af fortess nr. 4??

[Starbuck]

The minimal unit size in a battle is 50, so 19 will flee before the battle takes place and will have no impact on it whatsoever.
However, garrison or not, all attacking armies must still breach the walls before plundering the ressources .... just click on the "Siege Phase - Winner" part of your battle report to see the details round by round

Have fun,