Latins had a very detailed vocabulary for the military life: so "exercitus" is the generic word for army but "acies" is the technical term for an army deployed in battle formation.
Acies cames out from two experiences, the Ancient Generals card game of Lloyd Krassner and my computer card game Hoplites. The idea of this free card game is to simulate a battle between two armies represented by the player's decks. The cards represents the units (infantry, cavalry and artillery of course) and tactical options (rally, scout, etc.).
The rules are available here in html format: a french version of the game is also available (author is Emmanuel Delva), including both the rules and the cards.
Some optional rules are also available in this page (author is Emmanuel Delva).
16 january 2002: Added three new tactical cards images.
30 november 2001: Rules updated to 0.5.0
27 november 2001: french rules added
24 november 2001: Added Thoth module page