The best book-length treatment of political knowledge is probably What Americans Know about Politics and Why It Matters by Delli Carpini & Keeter. The thing is full of information, statistics, and analysis of why political knowledge matters in a democracy. A brilliant book.
So I'm looking at a 2-page table (Table 2.7, for those who care) and it has all these questions and the percent who answered them correctly over many many years, over many many surveys. They have two categories on this table: geography and social & political history. The highest percent correct for geography was locating Texas on a map (91 percent). The lowest? Where is Belize (4 percent). On social and political history, the greatest accuracy was on identifying the first president of the U.S. (93 percent), and the lowest asked what happened in 1066 (10 percent). Oh, the answer to the blog title -- Battle of Hastings.
I think it's kinda neat that at least 10 percent got the last one right. Honestly, it could have been a lot lower.
For those of you fascinated by political knowledge and what people know, I cannot recommend this book high enough. It ranges from what we mean by political knowledge to why it matters. I'll blog about it some more between debates, polls, and other presidential stuff. Hell, you could blog the appendices for a week and not run out of material.