A fortress is composed of one 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. Destroying each core is worth 2000 points, with the exception of Mono Core which gives 1000 points and Big Core which gives 4000 points when destroyed.
A false fortress is composed of two or more cores or a single core surrounded by Tanks. When they appear on the screen, screen continues scrolling as usual and you won't gain your fortress destroying bonus for destroying a false fortress.
The Mono Core is the core with the weakest offensive strength and least amount of hitpoints. It will only fire a single Lead shot at a time.
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. It's about as dangerous as Mono 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 since Panel Core firing at full speed will fire incredibly fast Sigs and sometimes be even more dangerous than Combo Cores.
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 there aren't any Round Cores and Big Cores around. Fortunately, Leads fired by combo core move slowly and a skilled player shouldn't have too much trouble maneuvering around Leads.
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.
A Big Core will fire both Leads at ship's general direction and Light Bars which are scattered in its front in 180-degree arc. 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. Note that Big Core cannot shoot light bars upwards.