Legio Quaternarius is a set of free wargames rules for ancients, played on a square grid. The author writes of the rules that they are:
Sweet and Legion! inspired fast Ancients Rules; we originally played them on a hex gridded table, but they should work fine using a Table with a 3″ square grid as well (for 25mm figures, mostly mounted on 60mm frontage stands). The rules are very ,uch of the “Rock, Paper Scissors Type”, being ultimaterly inspired by the WRG Ancients (? 3rd or 4th edition by this time)., which I played a few times but found too fussy for my tastes… plus those reactions tests; way too time consuming for what usually resulted in “Carry on as Ordered”.