Ironfell is described as 'time travel themed massive multi-player online real time strategy game. Travel back in time to plunder resources from the past, re-build your empire and conquer the stars'. There are more than 10 games similar to Ironfell for a variety of platforms, including Mac, Windows, iPhone, iPad and Linux. The best alternative is Civilization. It's not free, so if you're looking for a free alternative, you could try FreeCiv or Hedgewars. Other Ironfell like games are Worms (Paid), TripleA (Free, Open Source), Freeciv for Android (Free, Open Source) and Pocket Tanks (Freemium).
Freeciv is an open-source, turn-based strategy game based on Civilization II. Inspired by the history of human civilization, the game commences in prehistory and your mission is to lead your tribe from the stone age to the space age...
Worms is a series of artillery strategy computer games developed by British company Team17. Players control a small platoon of worms across a deformable landscape, battling other computer- or player-controlled teams. Represent entire series of Worms games.
TripleA is a free (under GPL) turn based strategy game based on Axis & Allies board game. A wide variety of map scenarios have been developed for TripleA (World War II, Punic Wars, Napoleonic wars, etc.
webDiplomacy is a multiplayer, online version of Diplomacy, the popular turn based strategy game in which you battle to control Europe; to win you must be diplomatic and strategic. You control armies and navies and attempt to overthrow the other world powers.