I think that another useful technique is to repeat previous levels to earn more Ilcoins in order to for upgrade more towers in later levels. This is just an opinion and I think it's going to be useful moving forward to later levels of the game.
Hmm.. interesting!

Is this a gray area or loophole you've discovered somehow?
Yessir! It actually somehow worked and I was able to collect coins from level 1 and used them until level 2. But for some reason, from level 2 to 3, the coins just reverted back to 150.