Nice Improvement
It's good to see this, i'm glad that you've taken the time to go away and improve on your last one so i'll do you the curtsey of writing a quick review.
It was playable although a few things till need to be ironed out: -
* The cannons all seemed to shoot randomly, rarely concentrating the fire on any specific group or target, it made it difficult and cost a few lives for me.
* After clicking a cannon there was no option or way to cancel your choice of it (without placing it), not a drastic problem, but certainly irritating - Also a similar complaint about selecting a tower to upgrade it, the halo was permanently on.
* I'm slightly colour blind, it was hard to see the difference between the colour of the placing dot, maybe next time in instances where you cant place it, put something like a cross through it as well, just to make it clear.
*More information of the types of towers before you place them would be great, fast towers sounded and looked great, until i saw that without upgrade they only did 1 damage, so the advantage was lost with them it seemed to me.
Other than those small things i dont think there is that much left to say. All that aside; Well done. It's a vast improvement on the last one i played and a great effort. Keep working on it and im sure you'll keep getting much better.