Since the game is designed so that no faction will ever conquer all of Calradia without the player's intervention, it is expected that the player will participate in many sieges during the course of the game. As a prerequisite, you will need to have a few men in order to besiege something - you are not even given the option if you don't have enough troops in your company. While a fortification is under siege, some of the usual entry options will be unavailable.