It's been a while since I played it so some of my memory is hazy, but the entire gameplay is inside the tower of the title, which comprises a number of fairly large floors (I want to say 10 of them). You're running around in real time isometric view as you would in games like BG or IWD, looting containers, finding secret areas, interacting with bookcases or machines or whatever (usually either to uncover more of the story or solve puzzles to open blocked off areas). When you unlock the next floor you'll be told the % completion for the one you're exiting (% of secret areas you found, gear looted, that sort of thing), for the completionist in you to obsess over.
There are enemies at fixed points on each level (I forget if they can move at all, but mostly I remember them being stationary, or at least very very short leashes) and when you run into them you're transported into an 'arena' mode where you fight waves of enemies coming from spawn points (usually doors or portals), sometimes with objects you have to protect or similar, and sometimes just to kill a set number of waves/enemies or survive a given duration. This is also real time isometric, with pause (or I think it was a super-slowdown) to issue commands. I'm pretty sure there you get advance info on the enemies you'd be facing so you can select party members or builds accordingly.
Most of the dialogue is with your party members from what I recall, though there are some NPCs to talk to in and around the tower. You don't create your own character - your character is kinda overseeing things remotely. There's I think 7 recruitable party members with max 4 in the party at a time. Each one has their own 'class', each with 8 skills of which you can choose 4 of, with 2 upgrade paths for each skill. You can respec builds and swap active party members between fights (I forget if there's a cost or restriction on either of those). There's also a crafting system for making and upgrading items.
So to say it's more like IWD than BG is true in that it's more of a linear dungeon crawl, and it's got the familiar isometric RTWP, but it's still a bit of a different beast than either of them.