The three levels of dreaming, each with a different timescale, were a marvellous way to build tension. In the snowy level, they're racing against time already, but then you remember that in the hotel level, they've only got a few minutes, and in the rainy city level, they've got a few seconds. And all that lasts for over an hour of screen time. The "race against time" feeling just kept on renewing itself.

The multi-level structure also made sure you kept thinking and paying attention, which is always good. The whole film was a puzzle, a maze, which I just about managed to stay ahead of – but it took effort, which made it exciting.

I also liked the fight scene in the hotel corridor where gravity was spinning around, so they ended up on one wall, then the ceiling, then the other wall, then the floor again. And putting an Escher staircase into a film was just cool.

The ending was very well-judged as well. An "and they woke up and it was all a dream" ending would have been terrible, but it all being obviously real would have been too easy a happy ending. Leaving it genuinely ambiguous gives you the best of both.