IANAL, and yes, you can not copyright game mechanics or ideas. This is why so many clones of every game exists. Because no one owns the rights to "a tower defense".
There is only one successful Tetris clone lawsuit.[1] Most Tetris clones are removed just because they are fairly aggressive with their claims (not necessarily because their claims are valid).
The successful suit was due to the playing field having the exact same dimensions as well as very similar visual expressions as regular Tetris outside of the just the game mechanic.