keystone, key, headstone
Meaning: the central building block at the top of an arch or vault
anchor, mainstay, keystone, backbone, linchpin, lynchpin
Meaning: a central cohesive source of support and stability; "faith is his anchor"; "the keystone of campaign reform was the ban on soft money"; "he is the linchpin of this firm"
Money is the keystone to success."
"Dead" secrecy was the keystone of all plans and confidences.