I'd like to know what attracted everyone to this castle theme. I'm sure we'll hear a lot of different reasons, so it's great to hear everyone's reason.
Well, I'm interested in history generally, so that's part of it. Also a factor is my romanticizing certain parts of the past... even when I knew a situation would have been absolutely terrible to live in, it's still cool to just think about the good parts (even if the good parts themselves are sometimes based more on fiction than fact). As far as why I'd romanticise and think about medieval Europe rather than, say, the Polynesian culture of the same period, there's probably lots of reasons. There's the fascination with fortifications that most young boys* have--whether snow forts, or tree cabins, or just making a fort out of blankets or a cardboard box in your bedroom. There's also the swords and armor, which I suppose appeals to the aggressive part of boys. Then there's the hero stuff, where you have to save the damsel in distress, fight the dragon to save the kingdom, etc. I suppose that when it comes to Lego, you can live out all the good parts of medieval times, build something that looks more like a real castle, etc.
*Just a note, I'm saying boys here, but I don't mean to exclude all girls. It's just that it's been my experience that, because of cultural conditioning and/or genetics, boys generally seem more interested in this stuff than girls.