Travel between levels and zones of the city is restricted and security check is required. Since every criminal offense is registered in a police database, checkpoints are pretty efficient way to keep offenders confined to their respective zones. If you try some funny stuff, the sentry-bot will take care of you, so to speak.
Concept from a personal project, sci-fi/cyberpunk story 'Butterfly'. It has been featured in 'EXPOSÉ 11' by Ballistic Publishing.