What do characters know about the battlefield?

Question

The PCs generally know where all enemies are and vice versa (except those who are Hidden). They also know the effects of all zones and other buffs that are in effect. This is part of the "all the cards are on the table" philosophy of 4E. They may or may not know the monsters powers, HP's, vulnerabilities, etc. depending on whether they make their Knowledge rolls (but they'll figure them out soon enough).

My question is, do the PCs know whether they have Line Of Sight or Line Of Effect to the monsters? My guess is that they obviously know whether they have line of sight, since a blocked line of sight is equivalent to Partial Concealment (-2 to attack) or Total Concealment/Invisible (-5 to attack).

But I'm not certain whether the PCs know know for sure whether they have line of effect (partial cover for -2, superior cover for -5, or no line of effect meaning the attack fails). Suppose there's a glass wall, or wall of force there -- do they know about it automatically?

Answer 1

Apart from RAW, I'd say it's much more climactic if effects such as you describe are discovered during combat, and not in the previous DM narration:

"You shoot an eldritch blast against the kobolds on the other side of the room. The blast reaches half the distance, and suddenly stops against a glimmering wall"

against

"You can see a glimmering wall in the field, at half-distance from your foes. The surface grants cover."

I might be partial to the first method, but in general, I think it's more credible if the characters discover these things by themselves in time, not beforehand. It also depends on how much do you want to use the surprise element. It's also a question of fairness, real or perceived: if the enemy shots pass the wall, but the character's shots fail to hit, you'll have angry players.

To summarize: I don't think there's a clear-cut general answer. Some traps/features will state if and how much can characters perceive about them beforehand. Others will depend on your judgment. For these, put yourself in the adventurer's shoes: do they have a high enough passive perception to notice the thing? should they immediately realize what's going on?

Answer 2

Line of Sight is self-apparent: if the PCs can see the enemy, they have it. In most other cases, cover should also be obvious, e.g. the enemy is hiding behind a rock.

As for your hypothetical invisible wall of force, I assume you have a specific encounter in mind. For a specific encounter, pretty much anything goes. A special, magical (duh) wall of force that is invisible. An area where gravity is negated. A patch of blood-hungry grass that activates when someone is hit while standing on, restraining everyone. Don't be afraid to use your imagination - 4e is very much exception based.

So it is perfectly fine to have an encounter with an invisible wall as a surprise for PCs. That said, this is very much a "gotcha" encounter. Haha, you've wasted a big attack, suckers. Definitely not something that should happen often. Allow your players a chance (however slim) of avoiding the obstacle: check their passive Perception (passive Arcana?), perhaps lowering the DC each round they don't stumble into it, as the monsters carefully walk around a seemingly empty patch of land.