The Saxon Hall is the smallest Keep and the only one made of wood. You can place military units on the tower for defence but not on the roof.
This is the smallest of the stone keeps. You can place military units on the roof and on its single tower.
This medium sized keep has 4 taller towers so it has more defensive capabilities.
The largest keep is also the strongest and most defensible with its tall strong towers and drawbridge.
*) denotes a former staff member.