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. It requires more than 20 person strength and Royal towers to have a chance of getting a capital. The royal tower gives a 20% attack bonus when attacking and or capturing a capital. The more royal towers your alliance has the higher the attack bonus you get..
There are other capital that can be found in ice, sands, and peaks. Each of those capitals will give the recruitment of the renegade for that specific kingdom, but they do cost that kingdoms resource which are charcoal, olive oil, and glass. When a server starts out their is one capital in green, two in ice one in sands, and then one in Fire Peaks. The capitals in green can grow along with ice and sands, peaks number of capitals will not change. There is a capital in green for every 5000 people or so and as those people go into other kingdoms there is a maximum of three capitals in ice, two capitals in sands, and one in peaks.
When you are going to capture any just spawned capital it is treated as a robber baron with some more "good" defenders such as Halberdiers and longbowmen and other lower level troops such as Archers and Macemen, but it should be easily captured with a simple attack. When you have captured that new capital, it will automatically expand to maximum and have level 3 walls and then the owner will have to build the palace, a structure in the capital that will allow you to recruit the Kingsguard defenders and attackers and/or (renegades in other kingdoms). Without a palace in a capital, you will not be able to recruit the Kingsguard troops or the renegades.
If you wish to see what a capital's troops numbers are when it has just spawned in, get an espionage of a monument that has just spawned in and think of it having around 200 men or so in it, as for the tools of that newly spawned capital it will have defense rocks and flaming arrows.
Also, if the person that keeps the capital changes from one alliance to another one, the previous alliance will automatically lose its right to recruit kingsguard troops the second that person leaves!