ProjectileSpeed affects how much reaction time the player will likely need in order to successfully dodge the projectile. I would make sure that the aim offset is somewhat high, but only so that the tower may occasionally miss. As well, making the projectile fast but the cooldown slow would make each projectile harder to dodge, while also making the whole tower somewhat easier to get past. Turning on gravity would likely make it so that I need to raise the towers to be higher than the player, make the towers closer to the player's path, or making the tower aim upwards to strike the player from above.