Google Earth is a real gem (Mac/PC). Apart from its primary function of keeping me from getting anything done, it's also a great way to actually see places that you're reading about. I just picked up a copy of Heavenly Intrigue about the death/murder of Tycho Brahe. The first thing I see is a map of where the castle is in Denmark, and it was quick work to find where I believe it must've stood. Actually, I'm pretty sure about this.

Here's a drawing of how it was, from Fred Ihrén's webpage on Tycho's Brahe's Castle, alongside what I found on Google Earth (click on image to enlarge):

Seems a slam dunk. Anyway, very fun zooming in/out/around to get a feel for the landscape. I put the whole island on the right. It's not obvious from that shot that I'd be able to find traces of the castle, but it's very sparsely settled to this day.

And here's the placemark file for Google Earth, for those interested.