Sammy
The area where Henry had gone to was located in the southern part of the main territory building. It was located within the territory barrier. Due to that reason, as long as a person was not stronger than Alicia, they would not be capable of breaking through the invisible barrier, and reaching that place.
Although it was true that they could also utilize another method in order to be able to break the barrier, but it was almost impossible to be able to find something that was capable of doing something like that. After all, something that was capable of dealing damage that was way above level 9 of the gold rank was definitely not easy to find in this stage of the apocalypse where the diamond rank had just began appearing.
Henry, Adam and the others had managed to enter into the main territory building simply because they were stronger. Additionally, even if the barrier had not allowed them to enter, they could have easily broken it.