Trying to pick the one single key system in Ashes of Creation is like trying to bottle smoke. While the Node system is the predominant system of Ashes of Creation, the Castle System and Castle Region mechanics are no less important. While Castles and Castles Regions have their own independent systems, they are also directly tied into the Node System.
I want to emphasize that Castles are primarily guild content. While there is a lot of content for members of different guilds working together (Node Sieges, Node Wars), the seizing and management of Castle is primarily a Guild and a Guild-Alliance theme.
Castle and Economic Regions
Verra is divided into five static geographical regions. Right now, the map we have does not show these regions, but we know the regions are fixed and static. Their size and location are not influenced by player actions and they cannot be moved, bent, twisted, or destroyed. This is not to say the people inside these regions are not influenced by player actions. The seat of power in each region is the regional Castle and the castle is ruled by a player character (PC) Monarch.
The Monarch is the leader of the guild in control of the castle at the end of each four-week siege cycle. Whether you like it or not, you will be living in a region ruled by a PC Monarch. The Monarch has some governmental powers including the ability to levy taxes on all Nodes in the region. Most of this tax money goes to the Castle and Regional Development, but a small portion of it can be removed by the Monarch and devoted to any endeavors the Monarch chooses.
Overlapping with the Castle Regions are Economic Regions, and since they overlay the castle region they are also fixed and static geographic regions. While we do not know everything about them, we do know that they operate hand in hand with the Castle Regions and represent the zone that the king has economic control over.
All of the castles are going to have proper names. So far we know of a Dünzenkell castle named Milnar, and an Aelan castle named Korgan.
Castles at Launch
When the game first launches and the players arrive on Verra, everything in the world is void with the exception of the Divine Gates, the Starter Towns, and the Castles. The castles are going to be inhabited by NPCs. In order for a guild to assume control of the castle they are going to have to defeat the NPCs. Should they manage to accomplish that, the Guild Leader will become the regional monarch.
Castle Region Tax
Just like there are taxes in the Node System, there are also taxes in the Castle Region. The primary difference between the two taxes types is that while the Mayor of a Node cannot remove any of the taxes for personal use, the Regional Monarch can remove a portion of the taxes to use however they see fit.
We do not know what percentage of the tax system can be removed, but we do know that a monarch that drains too much out of the tax system leaves his region open to attack.
Monarch Ruling Dials
Just like the Monarch can set a tax on his region, the monarch can also influence his region with different influence dials that affect crop yields, resource spawning, event spawning, and can unlock different types of buildings in Nodes.
Castle Support for Node Sieges
There are many ways that the Regional Monarch can assist during Node Sieges. A very direct way is marshalling his guild and marching them down to the Node to participate in the siege itself. A more indirect way is by using some of that tax money to support the Node with tangible benefits like paying for gates and walls, by putting siege schematics in the siege prep store, and by hiring NPC (or even PC) mercenaries to fight for the Node.
Benefits of Castle Ownership
Other than the political power and direct control a Monarch has over their region, each monarch has access to one of the limited flying mounts in the game. Each monarch will have a Dragon that allows them to travel from their seat of power (The Castle) to wherever they are needed in the region. This true flight time will be limited, as is a reward for being the regional ruler. This mount also has PvP capabilities, so it is going to be something to consider when sieging the Castle or one of that Castle’s Nodes.
Additionally, up to five officers in the guild will have access to Wyrmlings on Siege day depending on how successful the guild is at building their Castle Nodes during the Siege Prep Month. These Wyrmlings are flying mounts usable only on Siege day. More details to come later.
Given the complexity of the topic, I am going to defer you to the Castle Siege page under PvP.