Intellectual rights to game concepts

For a client I might be making a small puzzle game about getting a item out of a puzzle. The concept will strongly resemble this game :

http://www.alibaba.com/product-gs/355972089/Car_Park_Game_Brain_Teaser_IQ.html

But will not be about cars or parking, and we will make our own "levels".

The game will not be a board game, but a smartphone game (iOS + Android devices).

But does ThinkFun sit on the concept so heavily that I can't make a game like this ? And where does "the line" go to what is acceptable?

The game will be free, and we are a non-US company (based in Denmark, Europe) if that makes any difference.

Answer

http://www.copyright.gov/fls/fl108.html

Copyright does not protect the idea for a game, its name or title, or the method or methods for playing it. Nor does copyright protect any idea, system, method, device, or trademark material involved in developing, merchandising, or playing a game. Once a game has been made public, nothing in the copyright law prevents others from developing another game based on similar principles. Copyright protects only the particular manner of an author’s expression in literary, artistic, or musical form.

Material prepared in connection with a game may be subject to copyright if it contains a sufficient amount of literary or pictorial expression. For example, the text matter describing the rules of the game or the pictorial matter appearing on the gameboard or container may be registrable.