Pros:
Art style is very fun. Seeing a bit of BTP influence which is always nice. The particle effects are great (The weapon pickup flair effect is awesome!). Sound effects are good for the most part. My favorites are the weapon SFX - each one has a unique sound that really sells which weapon you're using.
Appropriately challenging. Difficulty is hard to get right, but you did a great job here balancing the spawn rate of enemies with the power of the weapons.
Tower defense + top-down shooter is a nice combination with lots of synergy.
Cons:
Took a while to figure out what the enemy spawn sound effect was - I thought it was part of the soundtrack. Not sure how you could clarify what the sound is - maybe show an enemy spawn and play that sound effect to show players what it means.
Missile launcher weapon isn't very useful since the towers already use it.
Powerup orbs didn't spawn until after I died the first time. Maybe a status bool not getting set at the start of the game? Probably an easy fix.
You can use each weapon forever without penalty. An ammo counter or timer would go a long way here (probably should give the default weapon infinite ammo). Adding ammo would also solve the missile launcher usefulness issue since you don't get powerful weapons for very long
Not clear at first that you control the turrets (I was holding down the fire button, so the towers were firing as soon as they reloaded). The easiest fix to this is to add a simple laser line from the tower to the mouse cursor. This will make it immediately apparent that the player controls the tower.
When releasing the walking keys, the character seems to move until the animation finishes. It doesn't feel great for a top-down shooter - would probably be best to immediately stop movement and blend the walking animation into the idle animation.
Fun Ideas:
Different towers - maybe some shoot on their own. Towers could have different weapons and could be damaged and repaired/upgraded.
More weapons - maybe some that are stronger against certain enemies and weak against others.
More enemies, mobile spawnpoints, bosses, etc.
Great entry! I look forward to seeing it expanded upon!