The main strength of this fruit is that it allows the user to become a living fortress, the inside of their body holding many openings, including cannon ports, portcullis and drawbridges; deeper inside contains various rooms, ranging from stone brick rooms for combat to well-refurbished meeting halls, complete with long-table, chairs and even decorations. The user can hold even living people inside their body, by shrinking their sizes as long as they are within a vicinity within the user. When the objects and people within the user reach a certain distance away from the body, shown by a pinkish layer of an unknown aura centered on the user's location, they revert to full size. In addition, the user's castle defenses can be deployed in a circular formation to give them a full 360 degrees of protection and attack coverage, as well as the potential to conduct decisive surprise ambushes.
As long as the user does not manifest castle features on their external body, the castle features of their internal body will remain concealed and unnoticeable. This has the advantage of lulling others into a false sense of security who think they are facing a single individual in combat who appears to be lacking armaments on the surface, only for the fruit's user to reveal the militia and weaponry hidden within. If the user does not wish to engage in combat personally, they can shield themselves by releasing their subordinates and let them do the fighting for them. The user can also use their body as a storage facility for recreational purposes, as they are shown to house food and drink for subordinates and any outsiders.
The user has complete control over the fortress in their body, and can telekinetically move around and create objects from inside as they see fit. They can also manifest features of a fortress on their external body, including tank treads. However, movement from the outside does not seem to affect the inside, as the Straw Hat Pirates did not feel any movement vibrations when Bege started walking back to the ship.
The user also seems to have the ability to make a miniature version of themselves through a strange fluid as a way to communicate with those inside, however, being in their own body, the user can still just talk in order to be heard. They are capable of being alert and moving around in the real world while also being in their own body.
So, far this Devil Fruit has displayed no direct offensive abilities that affect those outside the user. This Devil Fruit's full potential seems to be dependent on the presence of other people within the user's fortress body in order to use weapons and launch ambushes against threats located outside the fortress. Should the user be inhaling smoke or any similar substances, the insides would be filled with it as well, as when Bege was smoking a cigar, the soldiers inside were suffering from secondhand smoke. The fortress can also take structural damage such as by catching on fire, although it is unknown how this affects his body. Anyone can escape his body should they simply exit through an opening, so the user must monitor prisoners at all times or keep the doors closed. Other than that, the user is affected by the standard Devil Fruit weaknesses.
Capone Bege primarily uses his ability to launch powerful surprise attacks against unsuspecting adversaries. Holding his entire crew within his own body, they can fire guns or launch cannon blasts from his body, which would return to full size once leaving the boundary of Bege's ability. The troops can also rush outside of their captain's body via horses to overrun and surround their enemies. All of this results in bullets, large cannon volleys and cavalry seemingly popping out of nowhere from one man.
Bege also uses this ability to provide a place for him to have private conversations with other people. When he does this, he is able to appear from a liquid form. When his larger body isn't moving, it remains dormant, and his eyes glaze over, however, his body is still capable of walking when he chooses to do so. Having his targets inside his castle grants him a field advantage, allowing him to overwhelm his opponents by forming weapons and traps anywhere he desires.
Castle Tank (キャッスルタンク,Kyassuru Tanku?): Bege manifests tank treads around his leg area, allowing him to move at high speed, similar to Franky's technique. It is unknown if his legs are covered by the treads or transformed into them; the anime shows the latter. This was first used to escape Nekomamushi after capturing Sanji and Caesar Clown in his body.
The Yami Yami no Mi, Numa Numa no Mi and Blamenco's ability are similar to this fruit, as the four of them create a space inside the user's body where things can be stored. The main difference being the Shiro Shiro no Mi is a Paramecia type while the other two named Devil Fruits are Logia types.
In the banquet hall of Capone Bege's castle body, there are pictures of himself hung on the wall.