One of the popular rulesets for playing battles set in the ancient period is DBM, or "De Bellis Multitudinous", published by Wargames Research Group. These rules allow players to fight battles using armies dated anywhere from 3000 BC to 1500 AD. While the rules are designed to be used with any scale of wargames figures, at Group North 15mm figures are used as these are cheap enough to field an army while still having the "massed battle" look. DBM is designed for "fast play", which means you can play a game all the way through in a few hours. The figures are placed on rectangular bases called "elements" and the rules specify how many figures are used on each base for each troop type.
We have two regular DBM tournaments each year: the state championship "De Bellis Adelaidium" is played at Group North over a weekend and the club championship "Companion" is played over a series of Wednesday nights spread over several months. See elsewhere on this website for details of these tournaments.