2 are their only 2 neutral places in a 6 player game?
3 do castles /strongholds count as units for defense??? Or give you any defense bonuses? Or do they just count as mustering points and victory points?
4 if my exhausted army gets attack and thus can provide no combat points, can I still win the combat with a house card?
6.is this game any good?
2 Yes (The Eyrie and Kingslanding)
3 A castle or stronghold doesn't give defence. The homearea's have a garrison token, which stays until loosing a combat over that area. The garrison token provides 2 combat points.
If the embattled area contains a stronghold/castle, the only difference during combat is what a siege unit provides and some specific house card texts.
Yes, stronghold/castles are used as victory points and mustering.
Your routed (retreated) units provide 0 combat strength. An order token it that area with routed units however, can still give combat strength (like the +1 support or the defence orders). The combat is resolved normal, so with all the combat strength modifiers, including house cards. Routed units can't retreat again so they die when their area is conquered.
6 This is my favourite game, and many people like this game very much. It is best to play with 6 players and that can be hard to arange sometimes, since a game can lasts between 3 - 6 hours in my experience.