This is not XCOM, but it is a good game. It is not turnbased, but realtime with pause. You really have to play it modded but there are amazing mods available for the ALTAR games. It is incredibly hard but also incredibly rewarding to play. If you completed TOTD you can handle this. DO IT.
I still play this after all these years as no one has ever made a better civ game. Haven't checked, but if you don't have Crossfire on GoG you should totally add that too.
