The Hero Core Room is located at the very top of the Assembly Tower. It is a top secret chamber. When the platform brings the newly formed Hero to it, a Hero Core is fitted onto it's body. The core is then charged.
Only Mr. Makuro, creator of Hero Factory, is the only one allowed access to this room. Although since nearly everyone in the galaxy knows that heroes have a quaza core, this does bring up the question as to why no one else may enter the chamber. As stated in the HF movie, a hero core is what makes them who they are. Also, a core seems to not only be the heart of the robot but apparently contains their personality and memories as well.