Baldurs Gate's big bitmap engine works by storing a large surface in memory and blitting off of it as the player moves. Portions of the image cache are loaded from disk/storage as needed. Its actually one of the simpler engines to program because you take away to the need to think in tiles or polygons as far as drawing you're screen goes. The worst task is simply going to be managing you're loads of data into the image cache as the player moves so that the smooth scrolling effect is maintained. There is nothing rocket science about it.
Where the player can walk is determined by what I've always called a walk map and this is where some of the low points of a big bitmap engine comes in. A walk map is established for you're large image. Its basically a 2d array respresenting xy space on you're screen. A specific entry in the array coresponds to a location on you're image and contains a value that tells you you're walking constraints. For you're player to move you query the location in the walk map that he is going towards and then as he is "walking" shift the large image however many pixels one location represents in you're engine.
The problem is that since big bitmap engines are feeding basically, as far you're drawing code is concerned, nothing but a bunch of random pixels, there is no way to dynamically create you're walk maps. You would have to hand fashion them for each large image and you will find that such activity is boring, tedious, and prone to error. I thought I had read about Bioware having 2 full time poeple entering in data for their walkmaps and other stuff in the constuction of Baldur's Gate. Not my idea of fun!
Comparitively, using something tile based you can attach states to specific tiles and dynamically determine things like a walk map. In the end big bitmap Baldur's Gate like engines are easy to do but because every piece of location information has to be done by hand, you soon discover that making dynamic, really interactive environments becomes a real chore, especially as you expand the game world even a little. Not to mention, as I said above, you seriously cut down on the ability to reuse artwork. That said, unless you have people dying to do data entry and art for you, go for tile based or 3d. You'll find it much more rewarding anyway :-).
