Good choice to stick with a tried and true genre! I sometimes get too creative and end up with mess.
This is a solid game - or maybe a start of one. There's so much you could do! Adding sound effects adds a lot of life to a game, and it's easy. Also, flipping the player sprite horizontally towards the mouse would be nice too, and maybe changing the mouse cursor to indicate that the player shoots towards the mouse position. Oh, and flipping sprite horizontally would be good for monsters - unless they're all symetrical and wouldn't matter anyway.
And thanks for supporting arrow keys along with the usual wasd!
If you rate and play my game, please leave me some feedback! It's one of the best ways to improve. Thanks!