Brian's escape is made difficult by different types of enemies. Here some info about the different enemies Brian can encounter during his escape.
The prison is being used by the military to keep their prisoners and use them for experiments. Marines patrol the complex to make sure nobody escapes. It's your job to outsmart them. You can see their line of sight by the red cone of vision. Use this to your advantage and you might give brian his freedom.
Besides marines who have great vision, there are also dogs dying to catch you. Besides the fact that they can see, they can also smell you. Because of their keen sense of smell they are a force to reckon with.
While dodging marines and their hounds, you should also have a weary eye for security camera's. Make sure you don't stand in their line of sight while sneaking around the area. Even when there's nobody there, they might still be watching you.
Hanging from above they can see everything. Helicopters are the bringers of bad news. By using the darkness in your advantage you might have a chance. Try to avoid their spotlights and they will have trouble spotting you.