Overview
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.
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.
We also know that in order for the taxes to reach the Castle, it is a weekly event. Each node will load a tax wagon and it will set out from the Node to Castle and/or a designated Castle Node. The citizens of the region are unable to attack the tax caravans, but non-citizens can attack the tax wagons and loot the coin. This is one of the two different weekly events attached to the castles.
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.
Castle Sieges
Given the complexity of the topic, I am going to defer you to the Castle Siege page under PvP.