Benefit: 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.
Source: Core Rulebook