Noble Master Games now offers Age of Conquest IV, a new installment in their turn-based wargame series described as a cross between Risk and Total War. This is free-to-play, with one map included for free and the ability to purchase others for cash or in-game currency. You can find this on the Age of Conquest website, and you can have a look at gameplay in the official trailer. Here's word on gameplay and cross-platform support:

A player commands his armies in one of the many ancient to modern countries including the Roman Empire, the Greeks, Carthage, Sparta, Britain and the United States. From Rome to Celtic nations, one creates his own warring experience, wages colossal wars solo, against the AI, or takes on gaming friends in multiplayer games. One can form alliances and fight co-op style with the AI and other players for ultimate victory. The game supports cross-platform multiplayer matches against players on PC, Mac, Linux, Android and iOS (iOS pending approval).

Features:

• Turn-based grand strategy wargame with maps & nations from all over the world.
• Challenging AI for single player games based on a genetic algorithm.
• Cross-platform multiplayer & hotseat-play including co-op team games.
• Diplomacy, economy and population management
• Map scenarios including Europe, Colonization, Asian Empires, American Wars, World Conquest and more.
• Map editor and a central server to host and distribute player-modded maps.
• Hundreds of factions including the Roman Empire, Carthage, Persia, Celts and Inca amongst others.
• High scores, playing statistics, achievements and multiplayer ELO-ranking.