Left click units to select them, right click the game board to move them.
Left click buildings to select them, select an option in the menu to the right to build it.
Send harvesters to gems to gather them, then send then to the headquarters to earn money.
Buildings can only be built on concrete, concrete can only be built next to other concrete. Nothing can be built out of line-of-sight.
If the map sucks, simply reload the page to start over.
Oh, and you can't win the game yet. You're supposed to win if you blow up the enemy base, and lose if the enemy blows up your base.

t4kns is an entry for the 2008 java4k competition, meaning the entire game is less than 4KB big.
It was developed by Markus Persson.