Wargames Rules For Classical Warfare is a set of free wargames rules by David Wareham. They are designed for playing games set in the period from c. 500 B.C. to c. 700 A.D. The rules use an alternate
movement system and whole figure casualty removal. They are primarily designed for use with 15 and 25mm figures.
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Brickhammer is a set of free wargames rules for playing wargames using Lego and minifigs.
Starcrashers is a simple tabletop game of space exploration and combat. The author writes:
Star Crashers is a friendly game for 2-4 players, ages 8 and up. Each turn, players move their spacecraft around the table, exploring by flipping over Blip Counters that could be Space Pirates, Gravity Mines, Derelict Ships, and stranger things! When a player successfully deals with an encounter, they collect Credits as specified in the encounter. Once all the Blip Counters have been revealed and dealt with, the game is over- the player with the most credits wins. You can also try playing teams, pairing up with another player and combining your credits to see which team can earn the most!
War Galley is a set of free wargames rules for ancient naval warfare. It very cleverly uses matchboxes and matchsticks for play.
Tom’s Ancient Warfare rules are a set of free wargames rules designed to be played with — of all things — Risk pieces. The author notes that the bases may need to be larger if you use historical miniatures.