Steam now offers preorders for Age of Empires II HD, saying an updated version of the RTS sequel will be released on April 9th, and offering a 10% discount for purchases before then (thanks Jason and Glenn). An announcement trailer is online touting remastered HD graphics (though the system requirements are quite modest), multi-monitor support, the inclusion of the Conquerors and Age of Kings expansions, Steam workshop support, a new multiplayer experience, and Steam exclusivity. Here's word:

In Age of Empires II: HD Edition, fans of the original game and new players alike will fall in love with the classic Age of Empires II experience. Explore all the original single player campaigns from both Age of Kings and The Conquerors expansion, choose from 18 civilizations spanning over a thousand years of history, and head online to challenge other Steam players in your quest for world domination throughout the ages. Originally developed by Ensemble Studios and reimagined in high definition by Hidden Path Entertainment, Microsoft Studios is proud to bring Age of Empires II: HD Edition to Steam!