The verb 固める is often misinterpreted, as its primary meaning is "to harden, to solidify" and that's the first and only thing people learn about it. However, it can also mean "to gather, to collect in one place", e.g. things that have been scattered around. So if you encounter a sentence mentioning a relatively large number of things that are solid in the first place and that are 固められた, you can be sure they have not been "solidified" but gathered in one place or however you want to translate it.