Contra is a freeware modification for Command and Conquer Generals: Zero Hour real-time strategy game. It's a big project started in 2004. It adds many new units as well as numerous new upgrades, new general's powers and buildings. It also adds new sounds, maps, bug fixes, enhanced graphics, and other effects, as well as three new generals. There are lots of people playing in multiplayer.
Sequoia trees have a gameplay role. Destroying one sequoia tree will cause it to fall down and create a domino effect. The end result is pretty obvious - the middle area of the map is completely swept. Remember that you can always achieve a comeback when the enemy has middle control by destroying the trees.