In Old Town, there is a secret pipe that takes you into World 1-1 out of Super Mario Bros. It is located on a roof in the Southwestern area of the map.

Head to the southwestern part of Old Town, at the farthest building in that direction (see map)
It will be on the curved corner-like part of the building, and you'll have to pull it open, which takes about 5 seconds of holding your use button. You'll then be able to enter.
You're in. You have to jump up and press your "Finish" button on zombies while in the air to kill them.

The secret blueprint in this area is the Pyza Suit, an item in Super Mario Bros. that Mario uses. In Dying Light, it allows you to glide high distances across the map. It is located in a hidden, invisible pink block that you must hit with your head by jumping into it.
In the level, it is located right near the tallest green tower, near the end of the level.