Three other things I wanted to add to the discussion.
First, you note that you had lots of traffic but few comments. I read an interesting article the other day that essentially asked why you care how many comments a blog post gets. It all goes to the purpose of the blog. If the whole point is to create some sort of community, then comments are really important. If, OTOH, the point of the blog is to drive people to buy products, you're still on the winning side if the blog results in more sales, even if no one comments. Now this was written in the context of blogging for some commercial enterprise, but I think the lesson can still apply. If you are getting hundreds of people visiting and playing the game you created, I'd call that a win. Sure, people commenting and giving feedback would be great, but I think the sheer volume of people is in itself a very positive comment. You'll see great variations in commenting - e.g. the Brothers-Brick is probably the most-read LEGO news site (hmm, maybe GeekDad gets more volume? but they're not LEGO-specific, they just seem to run a lot of LEGO news), and lots of their posts will only get a smattering of comments (except for 'controversial' posts). Brickset, OTOH, gets lots more comments. Take the announcement yesterday of the new Cuusoo set, surely a big news story in the AFOL world. Brothers-Brick got 10 comments and Brickset got 129, and I don't think there's any indication that Brickset gets ten times the views that BB gets. I have no idea why the one gets more comments than the other. I do see that Huw (of Brickset) is usually very involved in comment threads (see my final paragraph below), but, then, the Brothers-Brick guys respond to comments as well, so who knows?
Second, you asked about how we promote Classic-Castle. Really, by now the community has been around ten years, and we don't really do much to promote outside of posting notices of the CCC and other contests around. I suppose we could do more to self-promote, but people interested in Castle LEGO pretty much find us by Google, by interacting with CC members who post castle LEGO MOCs on Flickr, by meeting CC members at AFOL gatherings, etc. Several of our members and even admins are involved in many different blogs and forums, so there is a lot of opportunity to refer to CC, etc. Basically the community collectively going out in the LEGO world is our promotion.
OTOH, if you were trying to launch a new site, you would have to do some marketing. Just to summarize a few of the thoughts in that post you can't access, before you even start marketing, make sure there's some stuff in place. There should already be some real content in place. Directly invite an initial group, either as co-admins of the forum or just as a founding team of beta-testers, and have them start some discussion threads. That way, when you do market and people click the link to the site, there will be something there already. It just doesn't work when you see an announcement of a new forum and you go to it and there is exactly one post (the welcome from the founder). Why would anyone go back? As to marketing, if it's a LEGO forum, try and post in the various LEGO forums and invite people. Before doing that, you should ask those forums' admins if it is okay to announce other new forums. Most won't mind as long as it's LEGO, but it's always good form to ask. Also, just know that you are going to get a much better reception if you first get involved in that forum. Make some real posts, comment on other people's creations, etc, so that people see you as a contributor to the community rather than just someone who is joining simply to draw their members away to some new community. Let's face it - if you and I make an announcement here asking people to check out our new websites, I'm going to get a lot more hits (at least initially), just because I've got a history here and people will check it out on the basis of knowing me better. In addition to marketing on LEGO sites, consider relevant non-LEGO sites as well. For instance, if I was starting a new website all about Star Wars LEGO, there are a ton of different Star Wars forums I could post in, or whatever.
Third, and here I'll just quote from the blog post you can't read, the way you as owner/admin of the site interact with your readers/members will greatly affect the growth of a community. You asked about how CC grew as a community, and I think part of that can be credited to a group of about 20 or so people who were active in the very early years (both those in 'official' admin or mod roles but also other forum members) fostering a great conversation. Of course it doesn't hurt that we've got the most awesome theme in the most awesome hobby, so there's always great stuff to talk about.
Anyway, here's what I said on that blog post:
Once people start showing up on your forum, you need to moderate the heck out of it. Now, I'm not saying that you should rule with an iron fist, deleting posts and banning users. I mean that you should be on top of the various conversations, putting in helpful comments, starting intelligent threads, and generally raising the level of discourse. You should regularly post cool MOCs in the forums (by yourself or things you find on Brickshelf). Give real feedback to others' MOCs (not just "that's cool"). Where there are violations of your terms of service, (e.g. if you say no off topic threads or no swearing), you should correct those quickly (though politely - always clearly tell people why a thread was locked or a post was removed). Keep an eye on your member list for spammers, as these pop up all the time. Mainly, though, it is the example of you and your team that will keep things going down the road you want. The higher quality posts you make and threads you start, the higher quality posting you'll get from your members. This will, in turn, further attract new members who will also strive to quality conversations.
Anyway, I hope those thoughts are helpful to anyone involved in growing an online community, whether this is founding an online forum, or just trying to get more comments and feedback on their site.