Avernum 4 has been in heavy rotation for me, with over 20 hours clocked in so far. I've learned enough from Spiderweb Software to know that Avernum 4's engine is an older version. At first, I felt I liked the way Avernum 4 played a bit more than the games on the new engine, but I'm changing my mind.
After setting the world-view to full-screen, turning on all graphics(it's not a terribly graphic-intensive game) and grabbing a cup of wanna-be-coffee, I dove deeper into the Avadon 2 demo on my Nexus 7 Tablet and Avadon:The Black Fortress on my PC.
I really wasn't getting a perfect read on how the games played via the Tablet, even though I've put more time into that version. After finally booting-up the PC version of Avadon 1, I feel the deciding factor was more about the touch interface than the game's engine.
Avadon:The Black Fortress is showing a marked improvement, to me, with the way the game functions, in movement, combat and interface. The additional interface options are an improvement as well.
It plays great on my 7-inch tablet, but shines on PC. For one, the graphics are larger and show a crisper, higher-quality in both world-graphics and interface design over the older Avernum 4. That's to be expected, but being more noticeable on a bigger screen makes a difference, too. Switching to a mouse-interface was a huge difference I found, as well. Not only is it quicker, even though I can tap and drag on my Tablet, but the point-and-click accuracy felt better. I think that's more of a universal design issue with touch-interfaces more than anything else.
What has proven to be a big aesthetic improvement for me is the ability to make the playable game-world fullscreen, with the party bloodbars overlayed in the top-left corner of the screen. It may be the immersion-factor for me, but such a small change made a huge difference for me. It took it out of viewing it on a T.V. screen and put me into the world.
Other refinements include being able to push the game-view around by moving the mouse to any side of the screen. You can also easily get an enlarged view of the area-map by holding TAB or get NPC and enemy names by holding SHIFT. Characters now have a junk-bag to sell-all at vendors as well as many other little touch-ups.
At first I missed how everything was calculated on a turn-by-turn basis based on my movements and actions, but the way enemies and NPCs keep moving in Avadon, even if I'm not doing anything with my character(s) is okay with me in the end.
Overall, I think it's only positive changes and improvements to the engine since Avernum 4.