How does Portals develop a sense of enigma or mystery?

One of the main mysteries established when watching this film is the setting the woman is in. From the opening scene, the audience is already questioning the setting with many signs pointing it to be a prison cell, some of these signs include: tally marks on the wall, the surveillance camera watching her and the door that opens to deliver her food. All these elements of the setting make the audience ask questions such as: "why is she in a prison cell?", "has anyone been there before her?", "who is watching her?" and most importantly, "where is she?".  Later on in the film after the woman seems to escape from her prison cell however the sense of mystery is continued with the woman running through hallways and up staircases with still no answer as to where she is. The woman eventually breaks outside to find she is on top of a building with many others surrounding her with the twist then revealed to be that she had never escaped, with the outside she sees in front of her being an image projected in front of the screen. This twist further establishes the mystery of the setting, leading the viewer to believe at first that she is in a prison setting, then moving to a city with many buildings, only for it to be something else that the film leaves the viewer to guess on.

Another of the main mysteries shown is around the character herself as over the course of the whole film, we learn very little about her identity. When she wakes up in the prison cell, she is wearing a hospital gown and discovers the barcode on the back of her neck shortly after. The fact that the woman is unaware of the barcode makes the audience wonder who did it to her, furthering the mystery of the setting, making the audience wonder if it was someone in the "prison" who did it to her or maybe someone else. She is shown to have a strict daily routine with her working out regularly, perhaps in preparation to escape from her cell. When she does eventually escape, she is shown to be able to think quickly when running from the guards, despite having little to no knowledge of how the portal gun she discovered prior works. With such little information on the woman available, all the audience can gather is that she is strong, smart and perhaps experienced with the portal gun, making the audience wonder if she has used it before, further questioning her identity.

Overall, the film has many elements of mystery to it, ranging from the mystery of who she is and where she is at the beginning, to if she has experience using the portal gun and where she actually is after seeing the big twist. The film deliberately leaves the audience on a cliffhanger ending, allowing them to ask questions about the plot and the story itself as that too has many mysteries to it. The people chasing her are never explained and what they will do to her if she is captured isn't either. It is assumed they know of their true location as after she jumps to the other roof, they stop to let her discover the twist. In that case, why chase her in the first place if you know there is no possibility of her escaping? It is questions like these that make the film so mysterious both when watching it casually and analysing it further.