I really enjoyed this video and wanted to share it. Modern Vintage Gamer talks about how the source code for many older games is often lost, and has to be recoded from scratch.
This happened to Diablo, the classic PC game. But in this case, a Playstation port of the game contained a few files that were a huge help with the reverse engineering process.
Definitely worth watching if you have 14 minutes and are into gaming or game dev.