There is a key in a hidden pathway on level 15 and you need to grab it before you can access the door. Once you reach the Platform you need to enter the crawl state and crawl through the wall to get the key. 
There is a key in a hidden pathway on level 15 and you need to grab it before you can access the door. Once you reach the Platform you need to enter the crawl state and crawl through the wall to get the key.