First Game "tower defence"

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Apple launched a new language "Swift", and since I've been using Ruby, I thought now's the time to start iOS game development.
I made a small app called "2d-tower-defence-Swift", and I'd like to share the code as well as what makes a great game.

It's a so-called "tower defence" game, which you are defending your 'valuables' while the enemy attackers are trying to invade.
To see what it's like, one of easier ones you can play on PC is Desktop Tower Defence (it uses Flash).

As well as many other good games, this game forces you to choose one action over another.
When you play the game(while enemies are reaching), you have variables like your money in a game.
Now the choice is, how you allocate this money.
Typically, you have variety of 'units' that will attack enemies. Choosing one over others is a player's choice.
Another easier option some of these games offer is level up, which will improve (for example) your money increase rate.
Depending on how well you allocate the variables to those choices, your game result in win or lose.