Shield Spikes: These spikes turn a Shield into a martial piercing weapon and increase the Damage dealt by a Shield bash as if the Shield were designed for a Creature one size category larger than you (see “spiked shields” on Table: Weapons). You can't put spikes on a Buckler or a tower Shield. Otherwise, attacking with a spiked Shield is like making a Shield bash attack.

An enhancement bonus on a spiked Shield does not improve the effectiveness of a Shield bash made with it, but a spiked Shield can be made into a Magic Weapon in its own right.