From Black Desert Wiki
A siege in Black Desert, is when group of players and npcs attack a castle. You can attack sieges with elephants, horses, guns, and cannons. Also you can hire NPC's to help attack and defend the castle. Not much information in known about this feature. It's not known weather you're require to be in a guild to participate in sieges or not.
A siege battle can have up to 800 characters actively engaging in battle 400 vs 400. However, it's not known weather NPC's count as part of that number.
Players can take the initiative to attack in whichever manner suits their strategy. Not only pushing and pulling around the main gate – players can also attack with artillery, put pressure on the side gates, or rush over the main walls using ladders.
These actions can be used to create a diversion or to attempt a different style of attack. Cannons can also be moved to new locations, although the movement speed is rather slow.
The defending army can also choose from a variety of tactics. Players can open and close the castle gates whenever they feel the timing is perfect. They can also destroy enemy cannons, and utilize barricades.
There seems to be some sort of leveling system during sieges. No further known information.
No known information.
The winning side of a siege gets to control how much tax and prices of certain products will be in some regions. However, it's not known weather this refers to only certain type of sieges, such as one that involves Military Guilds.
You can use a canon, bow and arrow, and guns. You are also allowed to use ladders to climb the walls. No other known weapons at the moment.
Cannons, ladders, barricades, and artillery are all created using player crafting professions. The armies must work together to build their supplies, then slowly wheel their cannons into position. There is no particular profession which is dedicated solely to crafting siege supplies; rather, each profession can create different components of the siege weaponry.
You can use horses, and elephants.
You can hire mercenary npc's to help attack and defend castles. There seems to be different types that can be hired.
This NPC uses a sword and shield.
This NPC uses a bow and arrow.
This NPC uses two swords.
This NPC uses a pole weapon. Possibly to defend and attack players on mounted units.
This NPC uses long spears.
Players can resurrect if they die during a siege, but it will take longer and longer to resurrect after multiple deaths.
You can see some siege action in the second black desert trailer at 6:00 minute mark.