Originally Posted by KasperFauerby
Well, there is the whole part about story and quests. All this is implemented using a scripting language also used for the npc dialogs. This includes controlling NPC moods, gaining/loosing quest items, setting/checking quest flags etc. Pretty much most of the actual game play - besides combat.
Then there is the level setup which is implemented using a fairly elaborate "trigger/trap" system. This system implements all the puzzles (levers/buttons), text messages, key/lock gameplay, environment transforming (for example: drain the water puzzle in UW1), special camera angles, special graphics.. and much more!
Then there are all the items. Which monsters drop what? What does the individual items do? What about magical items? Spells?
IMO there really isn't any "easy" way. If you decide to reverse engineer the original file formats, then you gain a lot of "implemented game play" each time you manage to decode one more piece of the files - but decoding it all is *hard*.
If you go for the "lets just reimplement it from scratch" approach then you'll quickly realize just how much data is needed for this game