Note: This page is taken from n.wikia.com.
(As written in the N in-game help)
There are two key differences between you and these guys; they can't move, and you can't fire deadly ninja-seeking rockets.This devious turret will fire a single homing missile at the ninja whenever he's in its line of sight, which is basically everywhere around it that isn't covered by terrain. Only one rocket will be fired by each turret at any one time. If the rocket explodes without killing the player, the turret will fire another if they are still within sight. The rocket travels rather quickly, comparable to the ninja's top speed, and has advanced homing software. The turret has a slight delay after it senses the ninja before it fires.
Multiple rockets are hell, as seen in a9-0: Wicker City (by Life247). Difficult terrain, lots of bounce blocks, or enclosed space also allows the rocket to do maximum damage. An extreme and unique example of this is shown in a 1.4 level, Brickman's 80-3: Full Circle, where one has to keep the rocket behind them all the way.
To avoid rockets, one needs to learn how it moves. It's difficult to explain in words, since experience is needed. Generally, if a rocket is coming straight at the player, they should stand still near some sort of wall or obstacle, and move aside just before it hits. Hopefully it'll loop into the wall and explode, without harming the ninja.
There is one known trick for rockets in the 1.4 version. It is used in 80-3 by top highscorers. If the player confuses the rocket and makes it move a certain way, it can loop through the tiling in the center, allowing the player to move quickly through the circle without worrying about the rocket being on their tail. This technique is also used to make KRADDAs - DDAs that keep the rocket alive throughout the entire course of the map.