If you want it to be a genuine tactical board-based wargame, you need to start super super simple with units, preferably without involving chance. Chance often completely ruins wargames (see: Risk). Also, the way you have it now, attackers get 2d6 defenders 1d6 means that the defenders will almost always lose. On average the attackers get a +4 bonus to hits (7 to 3), which with even a single mulitplier becomes 8. this means that a basic marbit is about even with a freaking gump if it attacks. Which is wrong in my opinion. I don't really have a better idea at the moment, but it really does seem imbalanced at the moment.