Can a creature make a diagonal attack on the other side of a secret door?

Question

I'm not quite sure how to resolve combat around a secret door. I guess it must be like a normal door, but I wonder. Take this example:

Red Circle is directly next to an open secret door, and Blue Square is on the other side one square south of the secret door.

Can Red circle make a melee attack against blue square?

If so what is the attack penalty?

Can Red circle make a close burst attack against Blue Square?

Is there any attack penalty?

Answer 1

Answers:

1. The Blue Square and the Red Circle can use melee attacks against each other.
2. There is no attack penalty due to cover.
3. Both enemies can make Close Burst attacks against each other.
4. There are no attack penalties due to cover in this situation either

Assumption:

• The Red Circle and the Blue Square are adjacent (there squares touch on the mutual corner without obstruction.

Explanation:

Despite the other answers, there is no cover in this situation. Page 219 of the Rules Compendium^DDI tells us:

To determine if a target has cover, choose a corner of a square the attacker occupies, ... and trace imaginary lines from that corner to every corner of any one square that the target occupies. If one or two of those lines are blocked by an obstacle or an enemy, the target has partial cover. (A line isn't blocked if it runs along the edge of an obstacle's or an enemy's square.)

If you chose either of the leftward corners of the Red Circle's square, all four lines drawn to the Blue Square's square are not obstructed. Therefore, there is no cover.

While it is true that the definition of cover gives as an example of cover that the "target is around a corner", if you look at the illustration on page 219 of the Rules Compendium you will see that in that example, the target around the corner is 3 squares away. In this instance 1 line drawn from the attacker's square to the defenders is blocked and cover is achieved. (I've given a poor ASCII diagram which hopefully illustrates the point)

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A

By the rules, adjacent opponents do not receive cover due to a wall on a mutual corner.

If the two enemies are not adjacent, then cover is granted as per the other answers given.

Answer 2

Yes, they can attack each other.

The red circle can make melee attacks against the blue square (and vice versa) with no cover.

For burst attacks, the penalty is a bit more situational.

Close burst or blast attacks will not take any penalty, since there is no cover.

Area burst attacks against the blue square may or may not be penalized for cover, depending on the origin square of the attack. If the origin square is on the blue square's side of the door, and there are no other obstructions, there's no cover to penalize the attack.

If the origin square is on the red circle's side of the door, the penalties vary depending on the location. There will either be a -2 for cover, -5 for superior cover, or no attack at all if there is no line of effect.

If the origin square is on the red circle, there is no cover - just like a melee or close attack.

Rules for determining cover are in the Player's Handbook, page 280 or Essentials Rules Compendium, page 219. Rules for determining line of effect are in the Player's Handbook, page 273 or Essentials Rules Compendium, pages 107-108. Pat Ludwig has also quoted some of the important details in his answer.