That top-down view makes you remember some classic games like Xenon 2.
Graphics are good enough and the number of enemies is really good. There are many different ones and your weapons will be powered up as long as you pass some levels and collect items.
Ad Nauseam 2 is an entertaining game that can give you many hours of fun. If like those spaceship games where you have to kill lots of enemies, Ad Nauseum 2 is perfect for you.