The difficulty people are talking about is not due to clever design but due to annoying 'features' (bad design). For example you can run a wave once and be unsuccessful, and run it a second time (with the exact same setup) and be successful. It's seemingly random.
It often depends on the direction your towers are facing when you start a wave, because they take sooo long to turn around and face the enemy.
Another design flaw is badly tiered difficulty of waves. For example on level 7, waves 1-10 are easy and can be beaten with 3-4 towers. Suddenly waves 11 and 12 are next to impossible, even if you've been saving the max resources per round. Now all of a sudden you need 6-7 towers just to barely scrape by. There was no preparation for this. Another example is often when you do scrape by those difficult waves, the next 8-10 waves will be super easy enemies that you faced earlier in the level. It jsut doesn't make sense.
The dificulty doesn't ramp up like it should: easy medium hard insane. Instead it spikes randomly in waves: easy easy easy HARD easy easy INSANE, etc.
Like other people said the controls are clunky. It's not a problem during the setup rounds between waves, but mid wave it's harder than it should be to set up a tower.
There's no backspace function like in Defense Grid. You cannot 'Undo' tower placements without selling them for a loss. 1 mistake can cost you the mission and you have to restart, making it very difficult to optimize your bases. This gets frustrating since the levels are SO long (45 waves at level 7).
Note to the designers: Do 10 waves WELL, instead of 45 waves SLOPPILY. It's just annoying how many hours it can take to finish 1 mission after you do get it on lockdown.
Another problem is enemies can walk off the screen, so you can't even see them. The map does not scroll.
For all of these reasons, the difficulty of the game is due to Design Flaws and Not clever design. I cannot recommend this game.