| 
T | 
towers and tower houses
 (mainly around 1540-1680)
 | 
K | 
simple keeps
 (14th and early 15th century)
 | 
| 
KK | 
developed keeps including chambers and courtyards
 (from around 1400-1540)
 | 
E | 
E-plan tower houses featuring more than one wing
 (16th and 17th centuries)
 | 
| 
L | 
L-plan tower houses featuring a main block and one wing
 (16th and 17th centuries)
 | 
Z | 
Z-plan tower houses featuring 2 corner towers situated obliquely opposite each other
(16th and 17th centuries)
 | 
| 
O | 
walled enclosures
 (mainly 12th and 13th century)
 | 
C | 
reasonably important castles built around a central courtyard
 | 
| 
h | 
18th century tower houses
 | 
V | 
castles of unusual plan
 | 
| 
m | 
much modified or modernised castellated structures
 |