I thought it was a really good concept with some really major flaws. Setting asside the asthetics of the game (which is odd for me, because asthetics are very important with games) and just focusing on the gameplay for a moment.
Style. This game had fun gameplay styleing. I liked the around and through world defence style. It was intersting to me, and I really think it could go to fun places.
Resource management. I like that you control your resource income vs. depending solely upon the game athor to decide how much you should recieve.
Menu: I liked the pause to upgrade option, this is something that is often overlooked as we clamor to upgrade our towers through waves that don't stop.
Everything in this game, all of the upgrades, all of the building is completed long before the 'boss' building is defeated. Infact I had its life almost gone one moment fairly early on in the game, only to look down seconds later to see it re-filled. I understand the desire to make a game last a while, but just making one item take an insane amount of time to kill is not the way.
-It got monotonous
-Ran out of upgrades too soon
-"Turrets" sucked and seemed useless.
And one last big con.. Your tutorial. You could condense all of that into a single screen which would be more memorable and far less anoying. The tutorial almost got this closed before I even started playing it.
I am going to skip over asthetics save to say, it has lots of potential just like gameplay, but it does need some work.
Overall some suggestions for when you put out another version of this, or for your next game.
1. Ditch the insanely long tutorial. Seriously major buzzkill.
2. Try making levels or altering the difficulty curve rather than making one building retardedly hard to defeat.
3. Add an in game help screen. Those of us that were bored into a coma with the tutorial may have missed (or forgotten) important information from it. Not to mention even if the tutorial was great sometimes people forget.
4. Add a wider variety of upgrades to counter the enemies ever upgrading forces. perhaps even giving the player alternate player attacks. This relates directly to suggestion 2.
5. And finally even if all of these are implemented I could still see monotony being an issue if you don't find ways to change things up some. Such as alternate world configurations, perhaps even dynamic worlds.
There is obviously inteligence and creativity behind this game, as well as skill. I think that much more could have been done with it. It is a good start to what could be an amazing game. And that would be my two cents.