With the release date for the game projected for the Q4, Cyanide has been pretty secretive with info regrading the game. They have done two interviews and one preview of the game so far but all of them were nearly a month ago. We all want some sort of update: new screens, more gameplay info, a hello from the forum manager, something.
This is everything know about the game so far(that I can find): Enjoy

Campaign
Campaign acts as a tutorial for multiplayer
The game plays through 1000 years of Westeros from years -750 through 299
There are 20 missions
Events that you play through: Nymeria's landing in Dorne, Aegon's Conquest, and Robert's Rebellion
Some events will be omitted but may be included in the future via DLC
The player may choose what House to play as
Units can be upgraded, such as a spy can be upgraded to have poisoned attacks
They said that they are looking into Co-op
General
You play as the head of the house, if they die then game over, even in multiplayer
Each house has a special units with powers
there are 4 types of units:military, diplomatic, economic, and underhand
Giants are stated to be in
Each house will play differently.
Multiplayer
A game in multiplayer can support up to 8 players whether they be player or AI controlled.
Map size will vary according to the number of players.
The main source of prestige is the number of alliances with towns and castles
You gain alliances with envoys and noble ladies but can be stolen by a spy and a rogue
Again it is pointed out that battleing is not the way to win, but underhanded tactics
Get resources and use them to recruit an army
Pacts allow houses to work together to achieve a common goal
but remember you can cause a revolt in an allied town
Houses
Targaryen
Bonus - Secret agreements go faster, knights get stronger and more armor
Special Unit - The Raven, flys around locates stealth units, can't attack or be attacked
Stark
Bonus - Commanders have two times more health and armor, men-at-arms move faster
Special Unit - The Direwolf, sees stealth units and boosts defense
Tully
Bonus - Mounted units are faster and more powerful
Special Unit - The Singer, can incapacitate another lone unit
Arryn
Bonus - Mercenaries are unlocked and the costs of military is reduced
Special Unit - The Theif, steals gold from an enemy town
Tyrell
Bonus - Marriage time halfed, seduction is faster, and armies cost less
Special Unit - The Maester, heals units
Baratheon
Bonus - Envoys are faster,bowmen are created faster and move faster
Special Unit - The Captain of the Guards, large field of vision and is a powerful unit
Martell
Bonus - Spy's field of vision grows faster, horsemen have a larger FoV
Special Unit - Dornishmen, elusive mounted bowmen
Lannister
Bonus - Bribing takes half the time and costs half the price, merchants carry more gold each level.
Special Unit - The Mole, "takes over" an enemy envoy, assassin or commander
Units
Great Lord - The Great Lord is the head of the house, so if he dies without an heir, you lose
Bastard - You get bastards if you don't marry soon enough. Bastards have a negative impact on the world, and you will lose 10 prestige points. You can legitimize bastards for a sum of gold.
Military
Mercenaries - men-at-arms, bowmen, and horsemen - can be bought and bribed, they aren't as powerful as your Regular army, and they can't siege
Regular Army - men-at-arms, bowmen, pikemen, horsemen, crossbowmen, and knights - can lay siege and can't be bribed, but they will run away without a Commander
Commander - cause your army not to run away, can level up and gain the "Fake Army" ability, and can be bribed
Guardsmen - never flee, can guard towns and castles if they posses they "Bodyguard" ability, are invisible until used, can imprison enemy assassins, rogues, noble ladies, and spies
Diplomatic:
Envoy - gains alliances to towns, castles, temples, ruins, and gold mines, carries peace messages to enemies, can be targeted by enemy spys or rogues
Noble Lady - can be married to a town or castle to create a blood alliance(more difficult to break/generates more gold), can seduce lone units
Economic:
Merchant - carries gold from alliances to your castle, can be killed and have gold stolen
Underhand:
Spy - used to make secret agreements, turncoat, verify your own units
Assassin - can kill lone units, end uprisings in towns, gain the "Poisoning" ability
Rogue - cause uprisings in towns, disrupt alliances, bribe units
How the Game plays
The point of any game is to collect prestige. There are five ways to collect prestige: diplomacy (by having the most loyal landlords), economy, warfare, obtaining support of the Septons(Sept give a steady increase in prestige), and completing secondary side missions. Prestige for all the families are displayed on screen.
Games are divided into two parts Peace and War. At the top of the screen is an indicator of how close War is. Negative acts move you closer to War and positive one move you further away.
The game begins in the Peace phase
The main point is to secure alliances, either a normal alliance(envoys), blood alliance(marrying Noble Lady), or an unbreakable blood alliance(marrying Great Lord)
Once you get an alliance with a village, they start sending you resources.
Eventually War starts. I think that it is pretty self explanatory what you are suppose to do. OH! Once War starts, you can't claim anymore villages so you are stuck with the resources you got :/
Here are a few other mechanics that I ran across but don't really know where to put them.
Here are a few things that you can do
Kill opponents messengers or resource gathers
Steal villages from your opponent, openly or secretly
Recruit units
Get married and have children/bastards
Make pacts, break pacts with players
Hire mercenaries
Bribe opponents units
Assassinate!
Seduction
Siege towns/castles
Fake armies
Turncoats(I guess this is your army betraying you)
Free prisoners
Imprison
Ask for ransoms
Cause uprisings