There are some considerable problems, as well as some minor ones. Here is just a few, which others may have put:
• The game's art is inconsistent. If this is intentional, I would recommend following in the footsteps of Quadriscope by Brian Kent, The Differences series by Ivory Drive, or Foxglove by Neovers. It is fine if there is variation, but it needs to be close.
• There is minimal support from friendly cannons. This is fine if you can dodge and attack, not otherwise.
• Collision boxes work sometimes. I've gotten pushed into an object multiple times and had to hope not to get hit and die as they pelt the character.
• Level 10 is not a good level to have the bottom of the screen damage the player. The contraption can shove the character into it using its cannon, guaranteeing death as you bounce between the explosions and the damaging bottom line.
• Enemies do not have every angle necessary covered art-wise. Wolves and human units need to match the character with directions they can turn. Idealy, they should be locked to angles with images, or it ends up turning 90 degrees to shoot something at 60 degrees.