What are the president's options regarding Guantanamo?

(I am not trying to stir up a political debate! Just looking for some info on the president's authority regarding Guantanamo.)

European asking. In Europe people often criticize President Obama for not closing Guantanamo. I always defend him and tell people he tried the best he could, but Congress refused to pass according legislation. Also, the states/governors refused to accept prisoners from Guantanamo in prisons within their state.

So my questions are:

• What precisely stopped him from closing Guantanamo? Congress? The states?

• Could he use an executive order to close it before leaving office?

• Being commander in chief, could he just move them to military prisons within the US?

Again, not looking for a political debate but rather a brief explanation (in layman's terms) of Obama's legal options in the matter. Thanks in advance.