The Middle Ages
Trails
Castles
This is a huge topic, one you can take in many different directions. The pictures here were chosen out of convenience more than for any special significance.
One thing you might do is concentrate on castles in a particular area: England, Germany, France, Italy, Spain. Or better yet, compare between countries. Is there such a thing as a national architecture, the way there is for cathedrals? Or is military necessity pretty much the same everywhere?
A pretty standard line of inquiry, but still interesting, is to explore the pieces and parts, the architecture of castles and the logistics of castle-building. You will find there is quite a large vocabulary associated with castles and there's a good deal known about it.
Another path to follow is daily life stuff. Did people actually live in castles? Were they somebody's home? Or were they a military fortress with a garrison?
Finally, you could explore the history of castles, how they developed and what sorts of things happened to cause castles to change in look and function over the centuries.