Hello,
Scottjones advise would be useful. Your units suffered damages and morale drops from the defensive towers and heavy archers and ballistae in garrison. They fled due to a low morale before they managed to destroy the fortress in all battles you sent me. They weren't lost, they left the battle. If the starting morale was bigger, or if they continued fighting for a couple more rounds with a morale below 50, you would have won.
During Fortress siege, all melee units plus the Battering rams are deployed in the front line. Swordsmen and Spearmen are the only units, except the siege engines, inflicting damage to the Fortress wall. Their number in fortress siege is limited, it is 3 times the Fortress garrison (the basic one). Only this amount will be attacking the fortress walls, while the the rest of the army in the first line waits in reserve to complete the places of killed attacking soldiers. For Fortress level 8, this limit is 19200.
Battering Rams are limited by the same principal, but their quota is calculated separately. They don’t occupy Infantry’s places, but their allowed number is equal to the Infantry number divided by the number of people required for Battering Ram recruitment. - For Fortress level 8, their limit is 548.
During Fortress siege, all siege engines (except Battering Rams) are deployed in the third line. The number of siege engines (without Battering rams), firing the wall, depends on Fortress’ level. The number of siege machines that can attack Fortress level 8 is 500, the rest wait in reserve and don't inflict damages until they substitute the loses.
The limits are available at -
http://www95.imperiaonline.org/imperia/game_v6/game/help/?section=military&lang=en&speed=2#table-military_145.
For more information, please review Global Morale, Battle Structure and Battle Mechanics at
http://www95.imperiaonline.org/imperia/game_v6/game/help/?section=military&lang=en&speed=2