Wow, I mean, this is a game with unique game play mechanics, and where the bullet hell does have a new tactical dimension now according to this game. I notice that while placing the blocks in according the line between hero and boss it is the most valuable way to survive from the bullets. Actually I would like to create a whole bunker from the pieces and then play as a grenadier from it. The boxes should somehow indicate the remaining hitpoints, while there are no change to know how safe it is to stay there.
You should make the "mana" more noticeable on the screen, while the player is watching all time the incoming bullets and the hero character, not the borders. The block placing range indicator is not noticeable enough also; the better way to implement the range should be adding the cursor on the mouse, which where is colored shield cursor image indicates the out of range placing, and colored means placing there has to enough range. The sword icon instead would indicate that the placement would cause damage to the enemy. I still hope you need to shoot to deal damage instead of create damage to anywhere, but its pretty much about how the players or the developer likes.
There are something new and unique in the mechanics of this, and also much potential to fun game play. I'm not the big fan of bullet-hell games, but this has something as in mechanics which may create a changes at people preferences.