What AC bonus does a shield offer?

All shields give +2 AC. Tower shields exist, but they are huge, wall-like shields carried with two hands. They offer strong frontal protection but are too large and too heavy for normal adventuring or fast-moving combat. These shields are mainly used by soldiers in formations or during sieges, not by wandering heroes. If you are braced in place or using the shield as an improvised barrier, it grants +4 AC.