The base-building, horde vaporizing Diplomacy is Not an Option left early access last year to a pretty warm reception by those who'd followed its development. In it you build up little medieval town-fortresses and use that economic engine to defend them from copious hordes of attacking fantasy invaders...
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There's now a free 'shareware' version of horde-battling RTS Diplomacy is Not an Option
Source: PC Gamer (opens in a new tab)