Yes and no. Part of the motivation is that the Period Games Rules Page is a damned useful summary -- a quick glance at the full sweep of period games, which makes it fast and easy to find things -- and I'd like to be able to recreate something like it. That implies a desire for a natural hierarchy, and the reality is that this domain *is* so hierarchical that there's a fair incentive to make it one.

We'll see. You're probably correct that I should at least allow things to be multi-valued, instead of a strict hierarchy, and that bottom-up tagging is likely more flexible than top-down organization. But I think there *are* clear conceptual "levels", and that it's worth acknowledging that in the schema design...