If you are interested in doing a unit study on cathedral architecture, and you live in Minnesota, I would suggest a trip to either the cathedral in Minneapolis or the one in St. Paul - we have been to both and they each offer wonderful examples of typical cathedral architecture.
Some more resources you may want to consider:
First, this YouTube video made by David Macaulay...
...and second this made for PBS movie by NOVA, that can be found by clicking on the following link:
Building the Great Cathedrals.
What a great way to study architecture - by studying how man has strived to build the ultimate buildings of worship for our Lord and Savior!