Here is a guide about Sieges, territories and siege equipment. The author will tell you how does it works and why do you need fight for territories. Information here can change because CBT still going and with every patch note something changes.
All territories that you will have to conquer will be at Northern continent. Each territory will have unique resource and about 1-3 territories for castle.
Each claimed territory will have an area of effect around it that will let the owners charge taxes to whoever placed their houses in that area.
To conquer a territory you will need a special item crafted from akhium. As you can see a lot of resources.
This video will show you how to claim a territory
So what to do when you claimed/conquered a territory. You need to build your castle.
For 100m around this sword, territory owners can demolish anything and claim crops without any penalty. This area is generally used to build the defensive structures like gates, towers, and walls to protect the sword during Sieges.
Better use of your castle
One of Korean clans build it in CBT4. Walls around territory, inside walls they placed houses with farms and small fortress around sword made from walls, towers and gates. Looks very nice. So what I wanna say it's you can build your castle as you want.
So now we will talk about Sieges.
In order to be able to declare war and participate in a siege you must obtain a special drop from the raid boss and place it castle area.
Look a this video it shows siege mechanics.
Each side can currently have only 50 players participating but XLgames announced 100x100.
Now I will talk about benefits of owning a territory. As you know you need akhium to craft and upgrade armor and weapons. Water for akhium trees can be obtained only on Northern continent so you will need to put your house there because its to much trouble and time to bring it back to your continent ( you can read about it in another thread 'Akhium'). If you are a big guild/clan you will need a lot of akhium for dressing your warriors so you can choose to put your houses on someones territory and pay them money and any time they want they can force you out from they territory or you can claim your own territory and live there and develop your force. It sounds simple but there is more to it!
The other thing that we don't know yet for sure. In castles you will be able to put your own Vendors/NPC who will sell crafted/dropped items and other things.
Don't forget that craft in ArcheAge is very important thing. It will build great economic.