This is definitely a good game / concept but as others have mentioned, it takes a while to figure out. In my first game I went to clear a line as quickly as possible, because the 'tetris side' of the game is most prominent / obvious at a glance. Of course I ended up killing myself because I was standing in the cleared line. In the next game I died to a monster because I wasn't paying attention to my HP. Basically it took a while for me to figure out the 'dungeon crawler side' of the game to keep myself alive.
I guess that's the challenge of combining two completely different games into one, how do you marry them together so that it feels like one game instead of two?
Maybe an in-game reason why the cleared lines are dangerous? (like if they flooded with water or something before vanishing) Maybe as someone else suggested, a build phase where you take turns between a block falling and moving your knight? I played with the move-separately control scheme.
You did a really good job with this, and it might just need a good tutorial or introductory level to help new players "get it". Love the art style too by the way.