If you are familiar with the classic arcade game, Rampart you'll instantly be comfortable playing Invasion. For those of you that haven't played Rampart here's what you need to know.
There are two main phases to the game. The battle round and the build round (this page has screenshots of each round). Each round is timed so act quickly to gain points and survive.
The goal in the battle round is to destroy as many attacking ships as possible. Each round the attacking ships will respawn. To fire your cannon(s) left click the mouse slightly in front of the ship you wish to fire upon. You are only allowed to have as many cannonballs in the air as you have cannons. When a cannonball lands it frees the cannon to fire again. If the ships destroy every block of your castle the battle is over and you lose.
The build round is a little trickier and this is where the game usually ends. Your task is to rebuild the castle walls using Tetris-like pieces before time runs out. Notice the timer in the upper right corner of the screen because it tells you how much time you have to rebuild the walls. Left clicking places the castle pieces and right clicking rotates them.
Some of the OpenGL rendering techniques used in Invasion include stencil shadow volumes, animation with vertex programs, vertex arrays, and particle systems. A Quake style console was also implemented to allow certain variables to be modified on the fly.
If you are interested in learning more about these techniques be sure to check out the included source code.
As you quest for the high score there are two ways to gain points.
During the battle round the decisions are simple. Destroy ships to gain as many points as possible. You get 10 points for single masted ships and 20 points for double masted ships.
The build round offers a little more choice. Each grass tile enclosed in the castle gives you 2 points. Tiles containing cannons do not count. Initially you'll want to place as many cannons as possible, but in later rounds you may wish to forgo placing a cannon in order to get more points.
In the early rounds you'll probably get more points from the build round. In later rounds the battle round will likely yield more points.
That's it. It's that simple. Have fun.
If you have questions send an email to