Tried it our for a half hour. It feels like a clean desktop tower defense.

The game as it is today is too hard for new players. I have played a number of these games before and I can barely get through the first few waves. It took me three games before I figured out which turret I need to kill the bugs with shields. The fact that the first level has the details of "Let us start easy here" and I am selecting "green" difficulty makes me guess that you have probably tuned the game for yourself. Sure if I play a handful of more games I will no doubt do better, but it wasn't fun to lose so early.

Remember that it is never fun to lose. One approach you could follow is what Kingdom Rush did which is that players unlock turrets as they play certain challenges. They unlock new turrets on the levels where magically the level requires the player use it and thus learn about it. By the end of the game they have explored all of the turrets and seen some epic battles because they can combine them efficiently.

The classic tower defense taught you the game with the waves systems. There was plenty of time to explore the UI while waiting around between waves if you didn't click the skip to next wave button. You got to build something horrible and still survive 10 waves so you got to experience "a lot happening" and were willing to come back. It was forgiving in its initial placement. For example the first wave that has a shield bug there should only be 1 of those bugs. So you learn the lesson to build one of those tower but don't take a big hit to your lives. Shame on me if the next time those bugs appear I don't have at least one of those turrets, but the first time I just had almost no way of knowing. And the second time the shield bugs appear I should be able to make it with just 1 lower level gun because the game is rewarding me for learning that lesson.

It felt like I was being introduced to too many different bugs one after another. When you click on bugs perhaps show the turrets that are best at countering, provide multiple ways for players to learn something. Optionally different music for each bug or at least on the "King" level would be a good idea.

As you make tweaks keep watching new players and see how sub-optimally they are playing they game. We don't spend the points as soon as we get them. We put towers in the wrong spot. Tower placement will be horribly inefficient. I would rather build something horrible and later in the game be completely overwhelmed at my poor planning, but grinning knowing that next time I will do everything so much better because of everything I have learned that first game.

Games are about teaching mechanics and systems. How do you want to teach the game to players? Via waves or via challenges or something else. The first five minutes should be magical and not frustrating. See the book "A Theory of Fun" for more on this topic. Some games even dynamically adjust the difficulty. If the player is doing badly you could slow the game down or introduce more time between the waves.

Optionally head down to Starbucks and in trade for buying someone their coffee let them play and silently watch them and write down everything that frustrates them and ask them to talk out loud what they are thinking. Very quickly you will see what you need to tweak.

The rest of these comments don't really matter. Figure out the above where the game is fun and then worry about the stuff below because if it isn't fun UI tweaks or bugs don't matter.

The main UI annoyance has to be the fact that I am forced to scroll the list of normal towers for no good reason. This is the new players main interaction with the application and with scrolling you lose the ability to memorize tower by its physical location on the screen (which is the $^$# one that can kill the bugs with shields again?).

The UI could easily be tweaked to not have this limitation. The list of upcoming bad guys (along with the wave number and time to next wave) could be placed above the map, the whitespace between the clickable spots could be reduced, rather than having a game/settings tabs at the bottom just have a settings gear at the top right which changes it to the settings and flip it to an 'X' when in "settings" mode.

In the Turret description there is a little arrow to the right. I have no clue what that means. Maybe speed? Perhaps use an different icon.

If you add a tiny bit of bright colors to the turrets to visually separate them that might be a big improvement (worth testing).

Random bugs:
- Occasionally I wouldn't hear the guns firing, only the soundtrack
- Can't rotate my ipad upside down

Edit:
Lastly if the goal was to make a "Desktop Tower Defense" clone it looks like you are well on your way to having a fun game, but currently there isn't any sort of hook, something that makes me want to tell my friends about it. Combine the teaching system of kingdom rush with the freedom of "Desktop Tower Defense" might be a hook to go after. Have various challenges where you have to build different configurations for example. That will change it from being fun to being something I tell people about.