Hi, we are sorry for taking so long to review your game. We actually played it yesterday but we didn't have enough free time to write the review until now, so here it is.
First of all, you chose really hard graphic limitations but it seems like you followed them with success.
Also, the concept of the game is really interesting and fun, simple platformer mechanics mixed with puzzle level designs and a bit of physics.
However, we found that the game controls for the keyboard were a little confusing. We are used to having the directional movement keys on our left side (e.g. WASD), and the action keys on our right side (maybe arrow keys, numbers, etc). Also, we wanted to point out that, as far as we know, the ZX81 keyboard didn't include neither the arrow keys or the control key.
We (which actually means Frog, because Panda was only watching him play over the shoulder), got a little stuck in some levels. Especially in the one that's called "Seems a bit easy". As you probably know by now, there's some kind of "bug" that makes the key item slide upwards to the surface that's above the player when the player releases it as it surpases it. Do you understand what we're talking about? The thing is, there were some levels, the one aforementioned above all, that were so hard to complete for us that we thought there might me some mechanics that weren't properly explained. We even thought that the so-called "bug" might not be a bug but a mechanic itself. We were unsure about that and we hope that you can clarify it for us. The double jump thing was also slightly complicated to get used to, since you can only double jump if and only if you press the space bar key after the first jump arc is completed. We think it might be a good idea to allow the player double jump regardless if the first jump arc is over or not.
We played through the entire demo, so we could get a grasp of the whole thing, and overall we think the idea is great and it is bound to be much greater with these few retouches. We're excited for you to release more levels and we'll be willing to try them once they're out.
Good luck on your project(s).
Also, we want to wish you a happy new year. Let all your dreams and wishes come true, and let's keep in touch!
❤️Frog & Panda