For a first time entry this is really good! The concept is extremely unique. It gave me some missile command vibes but with a tetris twist. I hope you continue to develop this after the jam with sound effects and a few additional visuals. For the UI as well, I would recommend putting vertical borders on the sides to remind the player how far they have to fill out horizontally. As I was playing, I thought I filled out a whole row but it turns out I was missing a few blocks to the sides since I didn't know where the borders were.