Bastion - part of the defences of a castle standing proud from the wall and giving a good field of coverage for defenders. (http://thescriptorium.co.uk/glossary.php)
From the World English Dictionary (http://dictionary.reference.com/browse/bastion?s=t):
1. a projecting work in a fortification designed to permit fire to the flanks along the face of the wall
2. any fortified place
3. a thing or person regarded as upholding or defending an attitude, principle, etc: the last bastion of opposition
Though this word's meaning has survived over the years, it has taken of a new role as being handy for creating metaphors. "He was a bastion of honesty, when he managed to keep his tongue still."
What are you a bastion of?