Ai Senshi Nicol is played in a linear fashion, with the player advancing from the first stage to the seventh stage in order. There are no warps or opportunities to skip stages.
Each stage can only be cleared by collecting the three crystal diamonds that are hidden in each world. While most crystal diamonds are guarded by a boss, not all of them are. The crystal diamonds can typically be collected in any order. Once the third and final crystal diamond is located and collected, the player's inter-dimensional space craft will appear to pick up the player and allow him to transport to the next stage.
Each stage has a unique set of enemies. Although the appearance of the enemies can change from stage to stage, the basic set of behaviors typically does not. That is to say, each stage has a common set of enemy behaviors which are adopted by a variety of forms. Once you identify the behavior of a particular enemy on one stage, you are likely to find an identify an enemy on a future stage with a different look, but the same behavior.
There are a couple of enemies which are common to all worlds, displayed below.
| Land Mine | Blue Flash Bomb | Red Flash Bomb |
|---|---|---|
 |
 |
|
| Damage: Contact - 2, Blast - 2 Land Mines remain undetonated until you get within a certain range of them. Then they begin to flash, warning you that they are about to go out, before exploding. |
Damage: Contact - 2 Blue Flash Bombs are indestructible. They will continuously bounce back and forth in two particular directions (left/right or up/down) and pay no attention to shots. |
Damage: Contact - 2 Red Flash Bombs are indestructible. They are a lot like Blue Flash Bombs, only they will alternate which directions they bounce in whenever they are shot. |
| Hotchkiss | Vega | Crystal Diamond Guard |
 |
 |
|
| Health: 10 Damage: Contact - 3, Bullet - 2 Hotchkiss float left and right about the screen while generally traveling up or down until they are off the screen. The will fire 8 bullets simultaneously from their cannons. |
Damage: Contact - 2 Vegas are indestructible walls that continuously rise up out of the floor, and drop back down inside. If you are standing over them when they rise out of the floor, they will damage you. |
Health: 5 Damage: Contact - 4 Although these immobile shields protect the crystal diamond within, they can still hurt you severely if you are not careful and run into them. Shoot them from a safe distance. |