The idea of a picnic is nice and fun for a turret defender! Your work on the camera following the wave of enemies and the option to see the blocked areas are good and very useful features for the player! Thumb up!
I suggest you to add a hold middle mouse button mechanic to move the screen (because edges hovering is really unfortunate for browser based games), to add a grid for placing turrets (or if you prefer, to increase the one existing), and most importantly give a clue to the player when he can't place turrets because he's blocking the way! (I didn't get it at all the first too-many times!)
Overall, for a jam, nice job! If there were more depth (like more turrets) I would probably kept playing since this games are very addicting to me!