[Ben_S]

In a game last night, I had a unit of Chaos Warriors in the watchtower. My opponent charged with High Elves. The rules for buildings say that we get ten models per side fighting. His Hero cut my Champion down in a challenge, while his Swordmasters killed about another three.

How many of my models get to attack back? The rules are clear that, despite suffering casualties, other models will 'step up' to take their place - so the three casualties didn't matter, but what we weren't clear on was the effect of the challenge. Since only the Champion can fight in that, and he is now dead, does this mean that only nine of my rank and file models can attack? Or do I still get ten models' worth of attacks on the rank and file of his unit (i.e. can someone still step up to replace the Champion in my ten models)?

 

[CaptainSarathai]

Yes. A model will take the place of your Champion (if the Champ hasn't attacked yet), just as if you were fighting in Rank-and-File in the open. However, you cannot attack any enemy models who fought in the challenge, as normal with the challenge rules. So you'll still get 10 models worth of attacks, but your only targets would be the other 9 models beside his Hero (and obviously, any additional wounds spill back into the unit).

 

[Ben_S]

Thanks, although is there anything in the rules that makes this clear?

(We played nine, but only because I was happy to give my opponent benefit of the doubt.)

 

[CaptainSarathai]

It's the usual GW "kinda here, kinda there" type rules, but the majority of it comes from page 128. If you use the glossary at the back of the BRB, under 'Challenges', there is a page number given for Challenges in Building Assaults. The relevant rules are:

pg128 - Allocating Attacks - last paragraph
Note that casualties are taken fromthe "back" of the unit as normal. If one side suffers casualties before it fights, extra models areassumedto step up and join the assault party in place of fallen comrades. So ten modelswillalways get to fight, assuming there are enough left to do so!

That's the big that explains that if he wastes your Champion in the challenge, you still get to 'Step Up' and fill in the Champ's spot. Assuming of course that the Champ hasn't already struck his blows (for instance, because he has an ASF weapon, or because he's a Character with better Initiative). The underlined bit is what's important here: 10 models will always fight. So you always get to nominate 10 models to attack.

pg128 - Challenges - single paragraph
A single challenge may be issued and accepted as normal, but only models chosen as part of the two sides' assault parties can issue, accept, and refuse challenges - you can't use the challenge to increase the number of models fighting.

The part where they explain that the challenge is fought as normal sends you back to the Challenges section of the book. Basically, they're fought exactly the way they normally are - only models engaged in the challenge can strike one another, etc etc.

The most ambiguous thing about Building Assaults, actually, is the step-up rule. In one particular game, I had a rather cheese-mongering opponent stick the following unit into a building:

19 Dwarf IronBreakers w/ Full Command
1 Thane BSB w/ GreatWeapon

For his assault team, he picked 9 'Breakers, and their Champion.
I sent in 9 Chaos Warriors, and my Champion against them. He accepted the Challenge with his Champ.
Once I murdered the Champion, he was entitled to 'Step Up'. He elected to add his Thane to the assault party, and proceeded to mutilate my Warriors.
There are no rules in the book which prevent this.
Why? Because there is nothing which says that only "Rank and File" models are allowed to 'step up' into the fighting ranks. Players have naturally assumed that models are assumed to step directly forward - if you kill the front rank of a unit, characters who somehow found themselves in the second rank will step up and become vulnerable.
In reality however, there is nothing in the book that says this is how it works, and in a building assault, the abstract nature of the "assault party" means that it is entirely viable to put that Thane into the Assault Party after the Champion is killed.
Now, if he were joining the party after his own initiative then he would probably have to fore-go his attacks (we've struck all the I5 blows, so your I9 Elf missed his turn), but there's not even an 'official' rule specifying that.

So yeah - figure that out, once.

 

[Ben_S]

That sounds right; I think it's this bit that led to my uncertainty:

only models chosen as part of the two sides' assault parties can issue, accept, and refuse challenges - you can't use the challenge to increase the number of models fighting.

But perhaps this follows from thinking that you nominate an assault party at the start of the combat phase and are stuck with them - but that can't be right, given that rank and file can step up. Presumably the composition of the assault party is somewhat fluid and always numbers ten, so as soon as one is cut down someone else always instantly becomes part of the assault party (assuming there are enough other models).