OverThrown is described as 'multiplayer turn based strategy game'. There are seven games similar to OverThrown for a variety of platforms, including Online / Web-based, Windows, Linux, iPhone and iPad. The best alternative is Civilization. It's not free, so if you're looking for a free alternative, you could try FreeCiv or C-evo. Other OverThrown like games are Dominus (Free, Open Source), UniWar (Freemium), WebDiplomacy (Free, Open Source) and Statebuilder (Free).
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...
Dominus is a free multiplayer browser strategy game. Grow in power by conquering other player's castles and making them your vassal. Vassals send their lord part of their income. Make everyone in the game your vassal to become the Dominus.
UniWar is a turn-based strategy game where players battle it out on the field with units of differents races, building bases and using the terrain to overcome their opponents towards victory. Several modes are available along with many maps to play.
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.