Excellent
Definetely one of the best defence games I've seen!
Graphics were very good, even too good! Atmospheric effects were a nice touch to complement the flag as wind indicator, and the fog level was quite interesting.
Gameflow was also good, always challenging, never boring, and the new elements were introduced with good timing.
Only a couple suggestions to make:
1 - Make some kind of indicator to warn when overshield is about to end. I had no idea on how much time I had left on mine.
2 - Explosions covered my shield, so at times I couldn't see where it was to block incoming fire.
Overall a darn good game!