Finally it seems spells cast on a bessieged castle by third parties can affect the two sides (it's I think a system like for targetting with "none" priority, the chance to affect an army is probably based on its number of units compared to the other one).
About a "no castle" rule I'm after all neutral : we may consider that if someone succeed to keep the isle a sufficient number of turns to make one he desserves to have it (also as the province is mountain/forest/farm and I think only the worst castle can be built by each nation, most are only able to make an hill fort or forest rempart there). The isle owner will probably be ganged by all who can, so allowing him to make a castle isn't a too big advantage finally.
Another point I think we should discuss : do I let Pharaex as he is, nerf it a little or a lot ? (if I let Pharaex like he is currently, d6/e6/s6 with gems the encounter may be really hard for the firsts to try, and it may end in a waiting game if nobody dare to be this one).
When his generals just do their jobs (very good thugs causing lots of casualties, but killable by any large army or army with mages) it's near to be impossible to kill the sphynx without medium+ level magic, and being very lucky with an ethearal crossbow shot or similar spell (as he has good mr, base prot 36, casts luck+be usually, and eventually earth or astral mr spell), and once players have the required levels/gear, Pharaex will be able to cast a lot of destructive astral combo spells like neither darts, gift from heaven, etc. Also, even if "independants are routed" in round 20, with all the generals and crushers destroyed, they may win if Pharaex is not killed (I had a very unlucky battle in a test game in which he survived alone 20+ rounds after the message and my army was finally routed...).
So I may... suppress his gems to exhaust him faster, and/or reduce his levels or not (but if I reduce astral without suppressing it he would be too vulnerable to magic duels) or both (I think E6/D6 but no astral would make him more killable by normal armies ; or I may let him S6 but reduce or suppress earth - death is more needed as the undeads he summons are the only chaff of the encounter). "Not" may be fun too (more a challenge to win the isle) but if he is lucky in the fight may make it untakable for a very long time (would be fun anyway if he remains the only "king of the isle" up to turn 60+ -an heckuva of scenarized maps with sphynxs

like indyependant would say).