I 'unno, cause they're special? Your previous comment had implied that there were no castles period. Anyways, if every village has castles, the specialness of the castles diminishes just a bit more. Hoshi used to be one of the few if not only places you could build one, and that worked just fine.
I think the utilities idea is cool, but what about paying the water bill? Gold is about inflated as dollars are atm... Even moreso actually.
Just how are Iwa and Kumo so special that the other main villages can't have castles? Sure enough, adding more castles may diminish their 'value', though letting Kumo and Iwa have them for no unique reason is a flaw in itself. They could probably be made more expensive to construct in turns, gold and gems as opposed to Hoshi, but other than that aside from the value-ridden sentiment, there's no reason to not make castles available in the other 3 main villages.
If it's about sustainability, then players should probably have to create their own means of providing the energy for utilizing certain utilities, such as constructing solar panels or wind turbines to provide electricity for the kitchen, and maybe constructing a well and a filter for a fresh supply of water for the kitchen and perhaps the bathroom ('Squeaky Clean' buff anyone?). Of course in doing it this way, certain utilities won't be available to all dwellings.
Then again it'd be easier to just make a weekly or fortnightly tax with figures based on the number of utilities owned rather than making dwellings self-sustainable.