the weps and towers need hotkeyes, xp was fine by me, esp when doing the garden lvls, with thr hand of steel fingers... I would just go with destroyer, its got aoe, and the only good thing about engineer is tower discounts. after a bit you need only build missles and barriers, if your not a destroyer, poison towers. I couldnt find out the reason for the freeze... it barely does anything.
there are items that regen mp per defeated enemy, only one I had issues with past lv 20 was the golems and dragons... last boss died and I was only at a quarter health when I got there.
upgrade your weapon 2x, sheild 1x, and go for the bottom row of upgrades, I started with either missles or secondary. stay along the valence ring(outer) of oranges and the station wont get attacked so much. take out oranges as you can, but you will need to upgrade your engine, going through the red star does nothing besides makes you an easy target for oranges and other ones. learn to move in a cyclic motion, as everything spins, up till you kill the last orange- I did notice that the 'system' got slower as I killed more oranges. oranges regen, so stay on one and pay attention to suroundings.
but really, there are about three different playing styles I have developed, and its eloquence and all political to my loving gaze. addictive, but is there a save?
after a long while of playing, s'round lv 35, the simple graphics shifted into my world, tunnelvision took up my eyes as my hands sweatily cramped into tuned reflex machines, and my rectangular prism was broken in its perfect negative space, blending as I became one with the razor and walked along the edge *drags on cig* I contemplate the path smoke must take in its vacuous march lungs... and my fingers hit P, and my avatar seems three dimensional, pulsing with bass.
 "Love"){kind=icon}
 "Bunny Flags"){kind=icon}
 "Pandemic: American Swine"){kind=icon}
 "Robotic Emergence"){kind=icon}
 "The Enchanted Cave"){kind=icon}
 "Hero's Arms"){kind=icon}
 "Harry Quantum: TV Go Home"){kind=icon}
 "Seven Deadly Sins"){kind=icon}
 "Johnny Rocketfingers"){kind=icon}
 "Ether War"){kind=icon}
 "Doodle Devil"){kind=icon}
 "Ray Ardent: Science Ninja"){kind=icon}
 "Doodle God"){kind=icon}
 "Run"){kind=icon}
 "Synapsis"){kind=icon}
 "Bowmaster Prelude"){kind=icon}