- Kill troops. The main and most important purpose of an assault is to kill troops in the target castle. An assault deals 5 times the damage as a siege wave. In an attack to take control of or destroy the castle, an assault is usually sent a second before a siege to significantly reduce or eliminate defenses in the city.
- Gain fame and purified resources. The second and related reason to send an assault is to get purified resources and gain fame points from killing the troops in the target castle. Every time you kill a player's units (also applies for lawless cities,) you gain fame points. The amount of fame points you gain is equal to the recruiting cost of the unit killed. For example, a ranger costs 150 wood to recruit. If you kill one ranger, you get 150 fame points. For each 4000 fame points you get in a single attack, you also get 1 of each type of purified resources.
- Loot resources. An assault also plunders resources of the target castle. Similar to a plunder command, an assault can also loot resources from the target castle. So, all the reasons to plunder a city also applies to assault. However, a non-castled city cannot be assaulted, but can be plundered.
Example: Here is an example of an assault report. I had previous scouted and saw that this castle has mages at home. Score! Mages are really good at attacking, but bad at defending, especially against knights. I sent this assault solely for the purpose of killing the mages. I sent one single scout with the knights and got all the castle information. I also looted resources as a bonus.