Hi Sven,
I played through again. This time I got up through Level 5 (Let there be light). More thoughts:
The tutorial did explain how the block towers work (more on that later). I was able to figure out the first several levels. I think part of the confusion comes from just how many different things the arrows can mean. For instance, a Block tower cannot be placed on a tile with more than two arrows. A better way to show this in the UI would be to make illegal tiles red and legal tiles green when the player selects a Block tile. Likewise, there are blue arrows even on the longer paths, with different shades showing the first path and the alternative paths the monsters could take. It might be easier for the player if the current "best" path was emphasized more - maybe a blue line going through all the tiles? - and 'possible path' was shown some other way.
I understand the Block towers better now, but I think they are a bit confusing at first. The problem is that they do two very different things: they remove arrows from an adjacent tile if the tower is placed on a tile pointing towards the adjacent tile, and if the tile is not pointing to an adjacent tile, they add arrows instead. The overloading of functionality makes them difficult to understand. I'd recommend splitting this tower up into two different separate towers - one that blocks on tiles that it's pointing to, and one that adds in arrows on tiles that it's pointing to. (This would require rejiggering the early puzzles at the very least, but it would keep the concept cleaner.)
The tutorial slides seem fixed, but I did find more blank slides after hitting the 'prev' button and then 'next' button on level 4, slides 1 and 2.
On level 1 (I think?) placing your first tower shows the entire tower and not just the 'roof' of it. Minor graphical glitch.
It's a little unintuitive that monsters keep going when they reach O hp. I see what you're going for with the 'overkill', though. I'd recommend adding some feedback when the overkill happens in game - maybe some tiny popup that reads "Overkill!" or something to let the player know instantly why their solution isn't optimal.
On Level 5, my first solution was placing the four towers symmetrically, first one tile away from the middle edge and then one more tile away from that. I pressed play and the first monster made it to the goal with too much HP, so I failed the wave. I hit the reset button near the play button and half the level disappeared.
I was unable to continue at this point. I tried to go back to the main menu and use the Level Selection buttons to return to this level, but clicking on those buttons only triggered the level title to appear at the top of the screen. The Continue button also led me back to the same broken level.