The other houses are of course still playable, but keep in mind that they have not been polished yet. We recommend playing the factions that are fully skinned for the best experience: House Stark, Joffrey and Tully.
This is a beta release, which means unfortunately not everything is entirely polished. It is based on the 'A Song of Ice and Fire' (ASoIaF) novels by George R.R. Westeros: Total War is a modification for Medieval 2 Total War. You should be able to move him around freely the next turn. Or, if you know how to use the console, you can select the assassin, bring up the console and type in 'remove_trait this' without the brackets. If possible, going back to an earlier save would be best.
The fix will only affect newly recruited assassins, so if you already got an assassin in your current game, you'll have to find a way to get rid of him first. Download the attachment here and put it into Medieval II Total War\Westeros\data\, say yes to rewrite.