Side scroller
Soldier with walk animation, also a sprite of his back for when he is holding the weapon
Engineer guys with walking animation.
Alien spaceship, 2 or 3 variations and no animation required.
Some kinda of weapon like a machine gun, it should be aiming up and also like in ~15-20° because it also shoots in an angle
The rest is just a one screen level,. 480x360
One building on each side of the screen , one is a junk yard and the other a hiring agency.
Missile in the middle, 1/3 of the screen height tops.
Background can be a big city on the other side of a river. Not so important.
A mother ship kinda like death star to destroy the planet after a certain amount of waves.
That's about it, whatever you can do to help really. Thanks.