Multiplayer has been added to Medieval Engineers! You will now be able to play together with your friends and either cooperate on constructing medieval castles or entire cities that you will have to defend from barbarian raids, or form groups and fight against each other. The initial multiplayer version supports all the features that are currently in the game, in both Creative and Survival modes. It is based on our current multiplayer from Space Engineers and it is not the new multiplayer layer - based on Raknet - that we have been working on lately (we still need more time to work on this new version).
https://www.youtube.com/watch?v=aMXrPs2Os7k
Full list of new features and fixes: http://forum.keenswh.com/threads/7359292
For more details about the multiplayer and the next steps in the game’s development, read Marek’s latest blog post: http://blog.marekrosa.org/2015/05/medieval-engineers-multiplayer_5.html