Palisade is a rather unique mod for Half-Life. It's not shooting people, it's not blowing things up. On the contrary it is a simulated combat world where you fight using bikes. When you move your bike along this grid (at which you can only turn 90 degrees at a time), you create a wall behind wherever you last were. The goal is to make the other player(s) either run into your wall, or their own. With everyone creating these walls behind them, you have to think carefully about how to take them out, while at the same time they are doing the same. It is almost like a complex dance. In other words, it's Tron, for Half-Life.