Ultratron is a retro arcade shooter developed and published by Puppy Games. It features top-down shooting with a very highly-stylised retro aesthetic. While one can expand on those two points, that's about what there is to it. It's a pretty simple game, and if you're looking for complex gameplay this isn't the place to look for it. What is here, however, is very highly-polished.
The game progresses through four coarse stages of a few levels each, culminating with a final boss at the end of each stage. The modes are kept varied by throwing in different stages that are essentially different game modes, for example, a "Dodge stage" whereby you must dodge away from a bunch of enemies without shooting them. The level variety does some way make up for the lack of enemy variety, but not quite. There's only a handful of true enemy types, although the actual number could be filled out somewhat since there are successive upgrades of others with minor cosmetic changes that mark their more powerful status.
Longevity is added by having an item shop in between levels, and the "pet" small robots you can get along the way are both useful and kind of cute, in a small robot sort of way. The game has a lot of character to the visual style in that way.
The look of the game itself is what I would describe as pleasantly retro. It has that 8-bit look, but uses a full colour palette and a variety of modern visual effects to add some jazz to the aesthetic. It comes together quite well, and with a complimentary retro sound design and chiptune-style music, the production oozes 'retro' from every pore. It's brilliant if that's the kind of thing you're looking for, though there are some that may be put off by the simplistic gameplay. If I had one complaint about the visual style its that sometimes the visual effects make it very hard to tell what's going on in the game sometimes. These distortion effects can be disabled in the options though, so it's not a big detriment to the game. If you have trouble with them as I do, you can just turn them off.
Nonetheless, if you are indeed a retro shooter fan, you'll greatly enjoy this game while it lasts, I think. At a low price point, it's worth picking up.