A fortress is composed of two or more cores. In order to destroy a fortress, you must destroy all of these cores. Additionally, whenever one core of a fortress is destroyed, the attack power of the remaining cores increases. For this reason, it is important to destroy the stronger cores first, and leave the weaker cores for last so that you do not become overwhelmed by the remaining cores.
Mono Core
The Mono Core is the core with the weakest offensive strength. It will only fire a single Lead shot at a time.
Slit Core
The Slit Core fires four Light Bar shots at a time. If you are shooting at it from the front, it is easy to shoot down the Light Bars which it attempts to fire at you.
Panel Core
The Panel Core shoots three Sig missiles at a time targeting the Zanac ship. If you fire at this panel from the front, it won't give you a lot of trouble, but if they are positioned on the side of a fortress, you must be careful to avoid the Sigs that it sends at you.
Combo Core
The Combo Core fire Lead shots in five directions targeting the Zanac. When you are not using a Special Weapon which can destroy Leads, it is quite a dangerous core. You will want to avoid leaving this core for the very end if it can be helped.
Round Core
The Round Core is responsible for firing the Beruo, a rotating shot. If you leave this core for the very end, it will fill the screen with Beruos, and you will have a difficult time hitting the core. This core should be one of your highest priorities.
Big Core
A Big Core will fire both Leads and Light Bars. As other cores are removed, the Big Core will fire more and more ammunition out, making it very difficult to overcome. Like the Round Core, this is one core that you will want to take out as early as possible.