Judging from the number of games I’ve seen at conventions, De Bellis Multidudinus (DBM) is probably the most popular ancients game out there. Unfortunately, it’s written in an obscure dialect of English known as Barkereese. Linguists contend that the form originally developed as a form of cryptography, allowing the Angles to the Saxons in the dark. Unfortunately, it also keeps gamers from knowing exactly what the author intended. As a result, many scholars have dedicated their websites to explaining it all. This site is full of diagrams explaining DBM’s concepts to those not fluent in Barkereese.
Here are a set of DBM Amendments for playing Zulu wars in South Africa.
Colonial DBM is set of free wargames rules that uses a modification of DBM to simulate 19th Century colonial Warfare