There are many things stopping N from reaching the exit.
The easiest drone in the game. It follows surfaces in a circular path, zapping the ninja when he touches them.
Zap Drones (2)
The Zap Drones that have an 'antennae' sticking out of them, do the same actions as the regualar drones, but if the ninja cross in front of it's path, they speed up, following him.
These drones are transparent, until there is a clear path between the ninja and the drone. It then fills up with red, and a small red line is aimed at the ninja. Shortly after, the beam becomes thicker, and if touched, the ninja will be killed.
These drones are grey. They seem harmless, until there is a clear path between the ninja and the drone. If that condition is met, it will rapidly shoot a burst bullets at you. If one hits the ninja, he will die.
These enemies are the only ones faster than the ninja. They sit still on a floor, until N lands on it. It then chases after him, at an incredibly fast speed. The only way to avoid them is to get off of the floor, by falling onto another, or jumping.
There are two types of turrents in the game, rockets, and gauss turrents. Each of them do not move, and only attack if there is a clear path between the ninja and the turrent.
These are circular objects, which have a transparent center. When a path is between the ninja and the turrent, a crossheir starts chasing the ninja. When it touches the ninja, the turrent shoots at him. It takes about 1/8 of a second to shoot, so they can easily be avoided if you have speed.
These don't fire until there are no walls between the ninja and rocket. They look like a gauss turrent, but the center is filled in. They launch a rocket, that is locked onto the ninja. It will keep chasing him until it collides with a tile.
Mines are stationary objects, that explode when touched by the ninja. They can be annoying when cleverly placed, and even avoid the ninja from entering an area.