Hey there, I hope this feedback is constructive.
I like the concept of having to control you square with the keyboard and also practice hand-eye-(mouse?) coordination to spawn the platforms that you need to make the long jumps.
However, I found the gameplay responsiveness unforgiving. The spacebar jump was not immediately responsive. My square would fall right away before being able to jump again. I would have liked to be able to spawn my yellow platform and then immediately jump again. But my square's momentum meant I would fall off my new platform, not being able to jump straight away. Pressing down constantly on spacebar - and holding - seemed like a very slow process to achieve a jump. This game would be much more fun if it's a bit more responsive and faster paced.
I've noticed this with other Unity games... why isn't there a pause (exit to main menu) type of functionality? :) After a while I just wanted to exit the game and not wait for the rest of my time remaining. :)
Which brings me to my last minor point: probably better to have made this a countdown from 25 seconds to zero...?