This Youtube account has a full Walkthrough guide on all three Versions of the game, being played on Namco Museum Volume 1 for the PS1. I will post each part. I will later edit this guide quite soon for the potential chance that these videos or account might disappear. When watching the videos, make sure you turn on Annotations, otherwise you won't see the info on how to get each treasure.

= Part 1 Original

= Part 2 Original

= Part 3 Original

= Part 4 Original/Ending

= Part 5 = How to find Another and Nightmare game room

= Part 6 Another Tower

= Part 7 Another Tower

= Part 8 Another Tower

= Part 9 Another Tower

= Part 10 Another Tower/Ending

= Part 11 Darkness Tower

= Part 12 Darkness Tower

= Part 13 Darkness Tower

= Part 14 Darkness Tower/Ending

Here's also a guide on Strategywiki. https://strategywiki.org/wiki/The_Tower_of_Druaga/Walkthrough