Halo Wars sold very well with 1 million units in the first month which is pretty good for any non Blizzard RTS TBH. But that was mostly off the Halo brand more than anything I imagine. Age of Empires III a year and a half after release had sold over 2 million units for comparison, tho it came out during a very low period in PC gaming sales. The high point of the series Age of Kings sold 2 million units in three months. Age of Mythology took 4 months to cross the million mark. Tho the PC games had very long legs.
And while I am looking at wikipedia other fun RTS sales facts. The fastest selling RTS of all time Starcraft 2 sold 1 million units day one, and over 3m in the first month. The best selling RTS of all time is still Starcraft with 11 million copies sold (that number may include sales of the expansion tho I am not sure). Worth noting that it is a subscription based game in Korea. Also Sega just released it's financial report and Rome 2 sold 1.13m copies as of March, which is within the 10% margin of error of Arstechnicas Steam sales 1.22m estimate. Ars also estimated that Age of Empires II HD had sold 1.16m copies.
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