I am making a tower defense game. The only player control in this game is deciding where on the game screen to place the weapons. Once the weapons are placed, they auto-fire projectiles. The projectiles follow a linear path from left to right. Enemies spawn from the right and travel right-to-left. If the projectile hits the enemy, the enemies health goes down. If the enemy reaches the firing weapon, the weapon begins to take damage. The problem is, none of these objects are colliding properly. The projectiles pass through the enemies, and the enemies pass right through the weapons. What type of collision do I need to set up on each object (weapon, projectile, and enemy) to make this work properly?
There is tutorial about whole tower defense game and it covers your problem with projectiles:
It can be downloaded from the marketplace: go to learn section and find “Strategy Game”.