On a couple of occasions you exit portals only to glimpse yourself entering the same one... it's like looking in a mirror only to see the back of your head. The collision between looking perfectly real and being clearly impossible is always affecting.

Gravity changes have more direct effects. Though you can't switch it at will, there are blue-lit plates dotted around which, when shot, reset that surface as the floor. At other times you're limited to special walkways which wrap around the floors and ceilings - these need powering, however, as you'll discover the first time a guard turns one off mid-run.

Above: On a couple of occasions you exit portals only to glimpse yourself entering the same one...

Unfortunately, the 'shooter' part of this first-person shooter is inspired by less up-to-date technology. There are relatively few enemy types, and while later ones look different they just take more damage. Once you've met them all, the difficulty only increases further by having different enemies appearing in quick succession, or en masse - the challenge gets more intense, but doesn't change.

The enemy design is wonderfully grotesque, but it's still more fun to fight the environment than the bosses. In one case you're suddenly trapped in a huge cube. Mother, the entity controlling the sphere, shifts the structure around you. Tommy can't shoot through the holes for the force field and all you have are a few gravity pads. What do you do? You think your way out.