This game is about as balanced as the witch scales in Monty Python. Most of the upper-tier guns are completely useless because they don't do significantly more damage than the lower-tier ones, the level difficulty to number ratio is often totally arbitrary, and it's just bad game design to balance a single absurdly hard level (58) with a weapon so overpowered that it makes the rest of the game a complete joke.
Movement needs to be restricted while attacking. At the moment holding down the attack key causes you to always do damage as long as you're in porximity to the opponent. Since this is necessary for them to damage you, it makes it impossible to lose. Characters also need to be prevented from moving through each other.
Also, which barrels are the ones required for that achievement? I can't find any castles with more than about 5 non-explosive barrels, none of which can be easily directly destroyed, and it is impossible to create them in the castle builder, so this achievement is either a massive grind-fest or completely impossible.
The castle builder is missing the small, non-important feature of... erm... actually being able to add people and features. Not being able to add any win condition or actually build a functional castle in the castle builder is something that really ought to have been found in QA testing.
Great game. My only complaint is that the lasers are too powerful compared to the other weapons... while the Tomahawk is only marginally stronger than the starting gun, the cheaper Avenger tears through every enemy until the very last two levels effortlessly, and can even bring down the second boss from half health in a single hit.
To elaborate on the invincibility "problem": you are nominally invincible because all shots fired by enemies are turned into coins and enough shots are fired off that they should kill any enemy you touch.
The reason this doesn't actually make you invincible is because 1) you can still receive contact damage from enemies that aren't killed by the stream of shots, such as the final boss and 2) the shots-to-coins effect does not apply to enemies that spawn after berserk mode has started.
A useful bug I found: press the attack button, then jump before the weapon has finished swinging. This will launch a second attack immediately. This is especially useful with the axe.
Is anyone else having trouble with the Tunnel Beast? At the start, it casts a move that uses 10 focus and makes it hop on the spot while having no other effect -- after that, the game continues to act as if the beast is in the middle of its turn when it's not, forcing a reset.