Surprise Attack

[Cumbana820]

Forgive this *ahem* basic question, but I would like some clarity on a rule:

I understand the rules of surprise, but my question is regarding ambush attacks.

In a few adventures, there are situations where "If the character walk into this room they are shot with arrows" or in the newest adventure I see a situation where if the characters walk past an illusory wall Kobolds shoot arrows through the wall at them.

So my question is, do I just have the attack happen or is this a surprise situation? Essentially, if the characters mentioned above enter that room they automatically have arrows shot at them, no surprise rolls. Same with the illusory wall. This is different then stumbling across a situation that may require a surprise roll, yes?

Thank you in advance for any and all help.

[Muze]

Personally I would not roll on these specific situations. When it is specifically written into the encounter. Also according to the rules as written the player characters would be unable to act for one round as well. However its your world if you think the elf should get a perception check because "he smells Kobolds" or some other reason then by all means have him roll it.

[Seven]

If the kobolds are aware of the presence of the PCs, than it's an automatic ambush.
If the kobolds are just sitting there waiting for something to happen, they might still get surprised and not take full advantage of the situation.
There is no way for the PCs to surprise the kobolds if they are hidden.

[Dimirag]

I would let the characters roll for surprise (modified by the ambush), they move, they get attacked (the kobolds are holding their initiative interrupting the moving characters) and they can or can't retaliate based on surprise.
The kobolds could be surprise if the characters (or one character) where outside of their awareness (invisible, moving stealthy)

[Solomoriah]

In the adventure mentioned (Endless Tunnels of Enlandin), if the kobolds are warned by the talking statue they cannot be surprised. If not, they might be, which would delay their attack by a round. However, they can still surprise the player characters after that surprise round runs out, meaning that the first round the kobolds are surprised, and the second the player characters are surprised and under attack. Of course, the kobolds might not be surprised, even if not forewarned, and thus gain a surprise round against the player characters right away.

The kobolds might also wait for the player characters to be more exposed before beginning their attack.