Zenix3D is, again, a simple arcade shooter. You fly a spaceship around a scrolling map and shoot enemies that appear from different sections of the screen. Your main weapon is a missile, though you can upgrade them by picking up power-ups that pop up during the level. In addition to blasting enemies, your main job is to avoid being hit. Though you have a shield to protect you, you are only allotted a single life. Blow up and you've blown it.
Since there's no story or really anything that connects the story together, Zenix3D can feel a bit too open-ended (maybe even overwhelming) for some players. You can play any level you want by flying into a door at the space port. Once in a mission, you just keep dodging enemies and shooting until the end - that's it. Adding to this "Too Open" feeling is the lack of any instructions with the game. There are no in-game cues that give you a clue about what to do (starting the game, how to shoot, goals, power-ups....) which actually caused me to lose interest early on. Now, this isn't something that will happen for everyone playing it, but it will happen to a few. At times, it almost feels like it is assumed that anyone will know exactly what to do. True, Zenix3D isn't the most complicated of games, but it shouldn't take more than a couple of seconds to figure out these basic concepts.
Where the lack of instructions really hurts is the level construction kit. This is a great addition to the game and something that could really make up for the otherwise short-lived appeal of the main shooting game. However, with no clear instructions easily at hand (you can find them online), it might turn away would-be level designers who simply want to make levels rather than have to figure out the tools.