The great Houses of Westeros had many other, smaller Houses that paid fealty to them. These bonds could be quite strong, reinforced by marriage. When House Stark sent out its ravens to call their Bannermen, House Tully was quick to lend their arms to the cause. In battle, the Sworn Shields would lock their shields together, side-by-side, creating a nearly-impenetrable wall, on which even cavalry charges could be halted.