It is mostly corridors in the sense that a hallway leading into a room with one exit and another hallway on the other side IS a corridor pretty much. Meaning the game is linear af. And it is trash mob delight, as most areas just feature the same monster/enemy over and over, maybe 2 of them if you get lucky. Most don't even have spells or special abilities, and the ones that do just cast the same simple shit (e.g. Hold Monster or Charm or Magic Missile, depending on the enemy).
There's corridors and then there's dungeons. Most IWD dungeon levels are some variety of:
Yes, one door in and one door out, but it's not a linear corridor and there'll usually be all kinds of different monsters to face. This level, for example, has Trolls, Acid Beetles, mad cultist clerics, spiders to name just the ones that come quickly to my memory, all of which have a different unique quality to their attack or set-up. There are also other features on this map, such as an interesting trap, quest resolutions and good loot. The only combat I'd class as being in anyway trash here is one or two too many trolls at point 7.
Where the game IS more like a corridor is some parts of Heart of Winter, the lesser expansion. That map is so boring I can't even find it by a quick google as no-one likely cares for it, the one where you kill a remorhaz, walk 10 feet, kill another remorhaz for the length of a single corridor map.
If you don't like corridors then I guess you'd hate the traditional cRPG dungeon design:
But if you prefer BG games, then I guess you're more used to your corridors looking something like this:
Oh wow, it even has really sharp corners and barely any 'rooms'.
Actually, on the top 70 list, some of the reviews said it has more mature writing than BG games. What a load...
The word 'mature' doesn't necessarily mean either 'more' or 'better', it likely just means you don't have to be interrupted by pre-made companions and the main narrative is about some mercenaries going monster hunting for both the lols and the glory rather than some uber-pretentious god fantasy. But it will be completely subjective, obviously, which type of narrative you prefer.
I am soloing, yes, which should make it more challenging and interesting, but it really doesn't. Haven't really had any difficulty plowing through everything.
The game is not designed to be soloed, per-se. The game is balanced around a party of six characters you make yourself. An adventuring party. If you went solo the first time you played then you might not be aware that you'll be getting 6 times the expected XP, and, obviously, it wont be too long before you're vastly more OP than the level of monster you're facing, so, yes, kinda shot yourself in the foot there.
However, I have to say, it takes a special kind of person to load up a game who's single biggest selling point is being able to build your own party of six adventurers and to then decide to go solo on your first run.
Further, and more importantly, you appear to give the game absolutely zero praise for letting you go solo. You don't seem to give two shits that the game actually has the ability to let you take on your adventures however the fuck you want. You appear to have ungratefully, and dare I say, entitledly, just said "I'm going solo! Thanks game for letting me! Game sucks, too easy!", which, if you don't mind me saying so, is a pretty retarded attitude.