This game is excellent and enjoyable in its overall quality,cute design,smooth gameplay and replayability value with upgrdades,medals and maybe the high scores.However,there are some problems like:
(1)The random spawning of monsters,the player could pretty much bump into them as new waves appear while the previous wave is not yet completed.
(Maybe there could be a mini map to show where the monsters are spawning from or that they spawn from outside the screen and an arrow should point out which direction they are coming from.)
(2)Monsters swarm the whole area as new waves become bigger,never mind about the weapons taking out groups of them,the mouse movement can make it difficult to control the firing directions.(A bigger map for more moving space or smaller monsterss)
(3)Like in Problem(2),as there is less space,it is easier for the player to hit the end of the screen when cornered and the movement is slowed down,affecting the speed of dodging(Maybe a wall or barrier could be added at the sides of the screen to prevent this.)
Anyway,an enjoyable game which could use more weapons(Drones and mines to add firepower,for example)and improvements.Good work!