At the most basic level, gameplay of 19 consists of automatically scrolling levels, where you avoid obstacles and enemies as you take control of the two protagonists.
The controls are extremely simple, but don't let that deceive you! Though things start off simple...
...they can get pretty hectic, requiring you to stay on your toes and constantly pay attention. Some levels really put your hand-eye coordination to the test!
Now, you are likely wondering what these dots next to each character mean. Those are health points; small dots are worth one point and large dots are worth three.
When the player and an enemy collide, whoever has less health will take damage. When enemies take damage, they will be defeated instantly, but players simply lose health points.
At the start of each level, you start with 10 health, divided equally across both players, but you can freely distribute this health between the two players.
When there is an enemy stronger than you that you can't get around...
...like here, where the upcoming enemy on the lower half of the screen has 6 health and you have 5, you rearrange the health between the two players as such...
...and voilà! Player 2 now has 7 health, enough to defeat the enemy!