Fireballs vs. Defended Locations

[Nail]

Again, easily that large. Taller than 4', wider than 3" (more like 5") in all of a half-dozen castles I've visited.

Warrick Castle has "cross" arrow slits, about a hand-width wide, and the cross was 2 feet by 3 feet.

Estimate, then, of 224 square inches.

Hey, someone from Europe: confirm/deny?

 

[schporto]

Where do you get that from? Air tight is not required....in fact, it looks as if RAW says the hole must be 1 foot square for the spread to go thru.

Well, I got it from the SRD. I may be wrong about it but here's how I got there.
Fireballs have Area: 20-ft radius Spread.
Under Magic Overview -> Area -> spread "A spread spell spreads out like a burst but can turn corners. You select the point of origin, and the spell spreads out a given distance in all directions. Figure the area the spell effect fills by taking into account any turns the spell effect takes."
I do not see a mention of 'line of effect' which is the only area where a 1' square is mentioned in these regards.
So why do you think it needs 1 square foot?
-cpd

 

[lukelightning]

I stick by the "Line of Effect Works in Both Directions" rule. If archers can shoot out, then spells can go in, albeit with more difficulty.

 

[Nail]

So why do you think it needs 1 square foot?

In essense: A "spread" spell goes around walls and obstacles by following a (modified) line of effect-one that can turn corners. A line of effect is blocked by a solid barrier. A solid barrier is defined as that which does not have a hole in it of 1 ft square or larger.

Here are the quotes:

A spread spell spreads out like a burst but can turn corners.

It then goes on to talk about how to count grid squares. The point is that, unlike bursts, spreads turn corners. They are otherwise the same.

Line of Effect: A line of effect is a straight, unblocked path that indicates what a spell can affect.

....An otherwise solid barrier with a hole of at least 1 square foot through it does not block a spell’s line of effect. Such an opening means that the 5-foot length of wall containing the hole is no longer considered a barrier for purposes of a spell’s line of effect.

 

[schporto]

In essense: A "spread" spell goes around walls and obstacles by following a (modified) line of effect-one that can turn corners. A line of effect is blocked by a solid barrier. A solid barrier is defined as that which does not have a hole in it of 1 ft square or larger.

Here are the quotes:
It then goes on to talk about how to count grid squares. The point is that, unlike bursts, spreads turn corners. They are otherwise the same.

Hurm. I'm still don't see anything in the area-spread description that mentions any line of effect. Just the 'turns corners' part. The effect-spread description mentions line of effect - but only in that you do not need line of effect to all areas. And the part you quoted about counting squares is in the Effect-Spread description, not Area-Spread description, they are slightly different.
(As a note - were you my DM I'd agree with you ruling this way. I'm just not sure its how I would rule.)
-cpd