The Capital is basically an 'outpost' which once captured allows the players alliance members to recruit kingsguard troops. These soldiers are the best RECRUITABLE troops. But the kingsguard defense (scout of the kingsguard and sentinel of the kingsguard) are the best in defense. There are other capitals in the kingdoms as well. There are capitals in ice, sands, and fire. Each capital allows the recruitment of the Demon/deathly horrors and the Ultimate Soldier for that particular kingdom, however they cost resources to recruit instead of coins (charcoal, olive oil, glass). There are a maximum of three capitals in ice, two in sands, and one in fire.
If you in possession of a royal tower and you attempt to attack or capture the capital it awards you with an attack bonus. Each royal tower provides a 20% attack bonus. The more towers your alliance has, the higher the number.