Quoted directly from the official forums:

"In the next expansion of King Arthur you’ll be able to conquer Britannia as the ruler of the Saxon Empire. One of the most important differences from the base game is the sandbox gameplay mode. The options are adjustable: after choosing from the various Victory Conditions, setting the AI aggression and the power of magic you can start your own customized campaign. This time only the Victory Conditions (among the Objectives) will have to be accomplished in order to win the game, but other numerous Objectives can help you achieve your goals.

As you might have already heard, this expansion includes a complex diplomacy system based on reputation. Depending on the nature of the relationship you can make diplomatic offers to the different rulers, trade with them or arrange that they declare war on each other. You’ll be able to deal with heroes or princesses as well who might join your court later if you had a fair relationship with them. When you deal with special groups – like the Sidhe, the Guild of the Outlaws, the Marauders and the Bishop’s Council – there’ll be some unique actions you can perform if you were able to reach a higher level of reputation. E.g. you can hire the guys from the Guild of the Outlaws to poison the enemy army’s supplies, pay a nice amount of gold for the Marauders to harass the enemy territory or “simply” call for a Crusade with help of the Bishop’s Council.
We added – among many others – a huge amount of artifacts and artifact-sets, lots of new battle quests, brand new unit types (like the Guardians) and new heroes, e.g. knights of the never seen order: the Champions of Beowulf.

Along with the expansions (Saxon and Welsh) we are still working hard on the base game. We’re trying to replicate that crashing issue some of you are still experiencing.
Hereby we’d like to take the opportunity to say a big Thank you! to all of you who have reported bugs here or via support mail. We really appreciate it, you guys are a great help for us in improving this game.

Some of you were also very interested in the release date of the Saxon expansion. Well, we already sorted out big bugs and technical issues, but it still needs some polishing. If we don’t encounter other unforeseen difficulties the Saxon expansion will definitely come out next month.
Till that we’ll be back to you with many info, arts and screens."