Basically, a military unit is just like a boat. Each turn they must be paid for with one unit from the treasury - if unpaid, they disband. Each unit tried to hit opposing units by rolling a six-sided die. Normal settlers hit if they roll a six, and armies hit if they roll a 4,5 or 6. Metalworking allows units to hit with one lower than they would normally, i.e. 5 or 6 for settlers and 3,4,5 or 6 for armies.
Defenders go first, unless one of the sides has military, in which case it goes first.
If a side is eliminated, it loses. So far this variation has worked fine. The player whose forces are being hit chooses which units are eliminated.