If you are using Chrome, Firefox or Microsoft Edge for Windows 10 (tested on version 25.10586.0.0) explore a virtual motte and bailey castle.
There are two versions available, a simplified version for slower computers and a detailed version for faster computers.
You may also be interested in exploring a Norman square keep similar to the one built in the reign of King Henry II at Dover Castle.
You may also be interested in exploring a Northumberland castle built near the Scottish border. An excellent example of a tower house with improved fortifications.