can't tell for other region, but knowledge here was hoarded by druids and associated orders (both written and memorized). when the romans finally managed to get in, the plan was to burn everything down so they could not steal that knowledge, with druids counting on the fact that they had that shit memorized and only needed to stay hidden until the invasion got repelled. except rome made a point to hunt down every last druid and the invasion wasn't repelled until generations later, so goodbye knowledge.
France