Very good
It was a short, simple game that reminded me of the term "Metroidvania"
The pixel art was great. You did a great job with this. Some diversity in the environment would have been nice though.
The sound was awesome. I loved the sound of the machine gun and the way that everything was muffled when the knight was submerged in water.
Difficulty was the only real problem I had with this game. Once you have the machine gun, the enemies are rarely a threat. I had to put what seemed like a lot of bullets into each enemy, but even the turrets didn't put up much of a fight. I never really felt like the knight was ever in danger all the way up through the final boss.
I only died once while playing this game and that was because I wanted to see what would happen if I jumped in the water. :P
Still, any complaints I have are very minor. I've played a lot of flash games today and this one was probably the best.