Total Conversion: Ys III vs. Ys: The Oath in Felghana
Here's the exterior to the Illburn (Alcaino) ruins. This area looses a lot of character by not showing the volcano in the background, which acted as a bit of foreshadowing.
This is the interior of the ruins. In OiF, you'll need to use the fire magic (obtained in the quarry) to light a few torches, Zelda-style, to trigger certain mechanics.
One of the outdoor segments of the ruins..
Adol happens upon Chester in the bowels in the ruins. In Ys III, he's located on some rafters and spies from above, but Chester eventually notices him. In OiF he just barges into the side room.
Here's one major spot where OiF diverges. In Ys III you never actually fought against Chester, despite him being (sort of) the primary antagonist. OiF changes that by putting Adol in a mano-a-mano battle with him. However, you're still captured after the fight anyway.
Adol is tossed into the volcano pits in much the same manner in both games.
Here's the volcanic area. In Ys III, it's only possible to survive here because there are some herbs growing on the side of the screens, which would regrow infinitely, although you could only grab one at a time. With the save points, this is no longer necessary. Also, in Ys III, progress is blocked by a lava pit which you need an item to solidify. OiF instead gives you an item that lets you walk across lava without harm.
Right before the boss fight, you're chased by these series of flares in both games. It's really stupidly handled in Ys III, because after killing the boss, you need to backtrack through this same room, but the flame's don't change direction, so you're running against them. You only need to run away from these once in OiF.
Here's the flame dragon boss. In OiF, repeatedly hitting it will cause its body to shrink, until there's almost only a head left.
At this point is Ys III, you have all of the items you need to escape. Once you do you come across Elena, who shows you a hidden passage and starts up another boss fight. There's none of this in OiF, because Elena's presence there didn't really make sense - if she's so weak and helpless, how was she able to get through the ruins without harm? Instead, you simply fight the boss right before you leave the lava area. However, you do run into Elena and Chester again when you leave the ruins.
And here's that boss, this big dragon thing. In OiF, it will crash into the platform, which will caused the layers to flip around and fling you into the air. The bottoms are spiked, which will also cause a lot of damage.
Now you need to go back to the quarry for some more stuff. I never liked this part of the original game, because it replaced the awesome theme music with something far more dissonant and irritating. In Ys III, it looks the same as the rest of the mine. In OiF, it's shrouded in darkness, unless you have get the item the old lady gives you. It's also here that you need the wind bracelet, which lets you spin around, and also lets you jump a bit farther.
This is another instance of some clumsy exposition. In Ys III, these miners are sitting around this treasure chest, who then explain the legends of the land, of the evil bad guy Demanicus/Galbalan, and the warrior who rose up to fight him. In OiF, instead you come across this room with a tablet, where some mystical force tells you the story. Yes, it's still cheesy, but a bit less awkward, and mystical talking voices are practically an Ys trademark.
The boss of the mine depths has changed quite a bit. In Ys III it's just a crystal that sits in the wall and attacks. In OiF, the fight starts off against a worm - one of its tendrils - before the crystal being emerges from the wall and starts floating around.
In OiF there's an additional subquest where you can escort Elena from the docks back to town. Falcom must've had a thing for these, because they added a similar scene to the Eternal version of Ys I where you need to escort Feena outside of the temple. It's technically pretty easy, since you only need to go through about three screens, but Elena can take maybe two hits before keeling over.