[andrewas]

OK. I think somewhat differently:

Each spell is a single entity with one damage constant. The random dissipation is merely graphical, the spell effects end simultaniously across the entire area. Not that it would have much effect anyway because the damage is applied per round and the differences are not greater than 1/10 a round in any case.

Seperate spells overlapping _do_ deal more damage. this is especialy evident with the high-level ones like Death Fog, but if you pay attention to the battle-text youll see it even with cloud-kill. the damages will appear more often than once per round if multiple spells are causing it. (This is easier to see when picking on a single enemy)

There is no effect upon casting the spell in an enclosed space. The area effect is a circle xx foot across, and the computer implements that. If you had a wall thin enough, it would be interesting to check whether cloudkill would take effect through it or not. but I cant, since im soloing at the moment.

Entangle is a bit different - you get a chance to save each round to see whether your held or free. This means that sometimes you will have dire rolls and get trapped the entire battle, and sometimes you will breeze through. Leveling up improves your saving throws, so in the latter parts of the game you will pretty much ignore this spell. Id guess the times you were caught quickly were down to poor luck. The spell works the same each time.