Personally I do not care for online multiplayer. I do not think games should get marked down because it is not available. X-Com has no online multiplayer but it does not make it any worse. It is up to the games designers if they implement it, and this review strikes me as marking them down because it is not there.
Given the low volumes of players across time that stay playing games online and the extra development work required I cannot blame them for not implementing it. I think a vocal few go on about it whilst most players are quite happy just getting to play the games on their devices.
As a reviewer, I try to review a game in the context of the audience of people interested enough to read a review. Castles is clearly interesting to some people who would really like to play it multiplayer (and I've heard that elsewhere, not just in the comments here). If the review were a five, I suspect some of them would immediately download it without reading for caveats, and would be very disappointed. A four is still a strong enough score that someone interested in the game will usually want to read more carefully to learn whether the game is likely to suit them. My expectation is that readers will interpret scores in the light of their preferences, so those who don't care about the issues identified in the review should treat the game as a "buy", while those for whom those issues are a deal-breaker may see it as a "don't buy"--those are the two review scores most people are really reading for, I think.
If you don't care about online multiplayer, this is a nice implementation of the game, but I'd still be just a little uncomfortable with a five, for reasons which might be more apparent from the images than the words. The visual impact isn't bad, but the design looks relatively flat--strong work from a solo programmer and nicely not distracting, but also not reflective of the merits some other games get from hiring professional artists. As one of the captions indicates, there's only one level of AI. The menu structure surprised me a bit and the tutorial leaves some important details unexplained. Even taken together, I'd be a little uncomfortable docking a full point for these issues, but that leaves the lack of online multiplayer in the role of costing the game a fraction of a point rather than a whole one.
So, those are some perspectives which shape my thinking about reviewing. They don't make yours irrelevant, but they balance it against the needs of other readers and my own intuitions. It's still great to hear from people who confirm the impressions of the developer--that makes me more confident in my tentative view that this was a daring but probably justified choice about where to put development effort.