CrystAlien Conflict

"Create an army, defend your base and defeat those scheming alien invaders! LEGO Mars Mission “CrystAlien Conflict” is a sophisticated real time strategy game. Each of the 30+ levels open with a movie which continues a narrative and introduces one of the toys." -Description from 4t2.ca Portfolio Website

The Lego Mars Mission CrystAlien Conflict game was made for Lego.com by 4T2 Multimedia (now known as "Amuzo"? Their work can still be found here) and was featured on Marsmission.lego.com. Since then, Lego has closed the Mars Mission site, therefore making it impossible to play the game there. Leveleditor6680 also has a site hosting the game for you (same link as below), so you can play it online or get the offline version from the downloads page.

CrystAlien Conflict was immensely popular, and was considered to be one of, if not the greatest LEGO online game of all time. It is a one-player strategy game where you play as either the Astros (Astronauts) or the Aliens in a quest for domination of Mars and the supply of Energy Crystals. Each faction has 9 levels to play, and a 10th "Conflict" level. Seven "secret" bonus levels may also be unlocked using cheat codes. In the Hive Edition of the game, new levels called "Special Ops" were introduced (3 for the Astros, 1 for the Aliens). Every level opens with an animation that explains the story of that level. Read more...

In 2011 a fan project called CrystAlien Redux was started with the goal to extend the CrystAlien Conflict game and keep it alive.
Since then there has been some development, and the first unofficial fan-made update to the game was released in 2015. This update fixed several high profile bugs in the game as well as a couple other changes and additions including an increased framerate, and the re-addition of an old level removed in a previous version.
A thread was posted to announce the release, which included a link to play the new bugfix version of the game, and a detailed change log.