Fun game, but a bit buggy
I found the game to be far too easy.
Once you get the health regeneration bonus outside of the level you can get enormous amounts of health (bug???) and you don't even need to use your shields anymore. The shield boosts, nuke, airstrike, mines all quickly become useless.
After a few money production boosts you can make enormous amounts of money that make every one of the levels too easy.
Things got kind of silly after about 1,000 money production boosts. I was making over 1,000,000 for each missile titan I destroyed.
I just had to destroy one tank and then buy a million time boosts to get through all of the waves.
I quit after I reached about 5,000 money production boosts with a highscore of about 20,000,000, which got erased for some reason. Took me about 2 hours to get to that point, but I could have easily gotten billions if I hadn't gotten bored of clicking the money production boost button hundreds of times (not to mention the time boosts to get through all of the waves as quickly as possible...).
I was kind of disappointed that nothing special happened after I beat all of the levels.