It does not state anywhere a challenge must be on neutral ground. Else Loremasters wouldn't be able to start a challenge or fight a challenge in an area they have previously staves up. Rangers would not be able to fight in forests and so on. It is
not a rule but instead a general curtesy many but not all fighters extend to each other.
Lots of debate about 2 man rituals in challenges have been on this bb but unless I am getting old and feeble there has been no official rule about using 2 mans in challenges. I know I have had to face it and many others have. I suspect it really is
up to the individual mage and their profession if they are ready to accept any consequences of getting killed for it.
You are right on about the sentinel however and I am sure it was just over excited young of Parrius trying to learn their abilities. We will defintly talk to them about interfering in a challenge.
I am just posting so that you don't confuse the young who are trying to learn to fight with some fabricated rules on how things work. Help Challenge spells it out very nicely.
Written by my hand on the 3rd of Midsummer, in the year 1100.