Siege is a common term in Stronghold games to attempt to force a castle's entire garrison surrender. There is a distinct 'Siege' mode in Stronghold 2 as well.
In these particular short games you have an army at your disposal, usually supported by siege engines and shields. Your main goal is to fight your way in the heart of the enemy stronghold and kill the enemy lord there. These fortifications usually consist of multiple defense lines, hidden traps and defensive ballistae/mangonels.
Stronghold 2[]
In Stronghold 2, the 'Siege' mode was introduced with multiple map layouts based on historic castles. With the patches, more castles can be added. You can either siege or defend them against the AI controlled troops.
In the lobby, you can choose from a list of castles. Each castle has its own template, a short background in history and sometimes an ancient picture of it. Then, you can choose whether to attack or defend.
When you attack, you must kill the enemy lord with your troops. He has several bodyguards around him and most of the time, traps will hinder your way. The AI is very passive, apart from moving some archers along the walls. Your troops cannot be reinforced any way. If they are all down, you are defeated.
When you are defending, you must hold off the enemy attackers with the castle garrison. You have full control over your defense, but you cannot expand it in any terms. The computer will launch his assault in a form of a regular invasion, erecting a siege camp then creating siege engines and laddermen. Defense is harder, as the AI has a little (but huge) advantage: it can recreate laddermen once they get killed. It also seems to know the location of pitch ditches, as its burning carts always target pitch ditches. Releasing rolling logs is also harder, as they must be manually instructed to do so for each logholder.
Conquest Trail[]
The Conquest Trail is a series of siege missions similar to the Path of War campaign. It leads you all around Europe, while sieging/defending historic castles. It has no cutscenes however and therefore it is less rewarding than the other campaign modes.