Aha, you meant that you play with max 5 cities. The use of the term castles made me confused. Sorry for that.
I would say that the strongest Civilization using this rule is Greece, for the simple reason that they have the 120 beaker tech Democracy from start. It is good to have such an expensive tech for free from the beginning, but obviously the strongest part of it is that they get +50% on Tech and Gold from the beginning instead of later. Also they start off with a courthouse which is valuable when growing a city from the start.
Zulu´s could also be good because of their attack benefits early on (I guess you are allowed to fight) and because of their faster city growth. If you have few cities it is important that you can utilize them alot.
Another strong Civ would be Americans partly because of their early good start with a GP, but also because of their strong production (factory x3 production instead of x2) which helps if you have to rely on few cities producing all your units and wonders.
Hope that helps. I guess Romans and Chinese are the two Civs that loose the most of their benefits because of this rule.
Want another person to join you now and then? I´d like to play with this rule as well, it could be fun. My PSN-id is Hertsh (if it happens that you play on the PS3).
Cheers.