Cores and Turrets Boss

Bosses in Video Games come in all shapes and sizes: giant monsters, giant robots, giant spaceships, and so on ... and then there are bosses who have no recognizable form whatsoever, taking an abstract (and usually mechanical) form distilled to two classes of elements: "Cores", the weak points you must attack and destroy to defeat it; and "Turrets", the weapons that attack you during the process.

These categories can overlap; it's common to see components that are both a weapon to deal with and a weak point to destroy.

This manner of boss is often stationary, such as being part of a defensive barrier or a vital element of a structure -- and in the latter case there's a very good chance it's also a Load-Bearing Boss.

Reactor Boss is a specific Sub-Trope, as the reactor is usually a core component itself and surrounded by various defensive turrets.

3rd Generation or Earlier (NES, SMS, GB, etc.)

 * The Contra series is known to include many of them. The first Contra game had one at the end of stage 1 (defensive building wall), stage 2 (and to get to this, more cores and turrets have to be blasted), and stage 7.
 * Zanac has one to six fortresses by the end of every level which consist of cores shooting stuff at player.
 * The Guardian Legend's first boss also is a boss with several turrets shooting football-shaped stuff at you.
 * Journey to Silius has one at the end of stage 3 which shoots tons of lasers.
 * Mega Man 2 has Buebeam Trap which can become Unwinnable if crash bombs are wasted. Then there's Square Machine in 4 and 'Rounder 2 in 6.
 * In 2, there's also Picopico-Master where the boss fight where multiple tiles merge in pairs and then chase Mega Man.
 * Silent Assault for the NES has the 1st boss consisting of 2 turrets which fires some of the slowest (needle-shaped) projectiles ever. Then the second boss which consists of 2 gears and the fifth boss which is a giant computer with a mouth.
 * Sunsoft's NES Batman game features a boss that pretty much essentially boils down to forcing the caped crusader to punch a security system to death.
 * The boss of each stage in the NES Legendary Wings is a moving wall with a core that opens and closes, and turrets that shoot eyeballs and shrimp-like things. The Final Boss is a teleporting core surrounded by four turrets, similar to the final battles in the later Mega Man games.
 * The Cheplin airship in Adventures of Tom Sawyer, which is also an Unexpected Gameplay Change.
 * A more obscure game for the NES, Jackal, consisted almost entirely of this kind of boss.
 * Metroid, of course, has Mother Brain. In most of the games she's appeared in, she just sits in a tank and lets her turrets and Rinkas do the fighting for her.
 * Iron Tank has the Long Range Turret bosses and the base defense walls.
 * Star Soldier has the Big Star Brains.
 * Bosconian is one of the earliest examples of this type of enemy, but not in a Boss Battle: you had to search every level to find the bases and destroy each base's core or the ring of six turrets surrounding it.
 * The Xevious series has the Recurring Boss Andor Genesis and its Final Boss variants.
 * Blaster Master's third boss is a room full of square teleporting mobile turret-cores that become faster and more agressive as the battle progresses.
 * Most of the bosses in Super Spy Hunter.

4th Generation (GEN, SNES, etc.)

 * Sonic 3 and Knuckles has a weird core miniboss at the end of Death Egg Zone act 1.
 * Tank Force, the sequel of Battle City, has a final boss which is compromised of four 3-barreled turrets which fire projectiles at fast rate and speed, considering the genre of the game.
 * Halberd's Reactor in Kirby Super Star is a good example. The core itself is actually invulnerable to Kirby's attacks; you have to lure the laser turret that occasionally appears into aiming at the core instead.
 * Too bad Kirby doesn't have the Laser Ability from Kirby's Adventure anymore, huh?
 * The CPU/Attacker/Defender inside in Final Fantasy IV.
 * The overwhelming majority of bosses in the Metal Slug series fit this trope, though some are Colossus Climbs.
 * Forgotten Worlds has the God Rah (4th boss), which is a giant egyptian boat with several turrets, a closing-opening "door" and the hawk-headed god itself serving as the core one needs to shoot.
 * Thunder Force II had free-roaming overhead stages in which the objective, much like Bosconian, was to seek out the many-turreted enemy bases and shoot out their cores to destroy them.
 * The recurring final boss of the Raiden games is a space fortress with a red crystal core guarded by multiple turrets and sometimes smaller crystals. The crystal itself typically unleashes Bullet Hell on you for its final attack.
 * Zero Wing does this with the reactor boss at the end of Stage 5.

5th Generation (SAT, N64, PS 1 etc.)

 * Star Fox 64:
 * Bolse is a prime example. First, there are shield generators protecting the core and the few fighters present, and once the shield is down, more fighters swarm out, followed by the appearance of the core (and the Star Wolf team, if they weren't defeated at Fortuna). At first, the enemy ships are the core's only defense, but each of the core panels turns into a laser turret once destroyed.
 * To a lesser extent, the Saucerer, which is primarily a carrier ship but also has a core with a giant laser weapon which must be destroyed in order to save the Katina base.
 * In Breath of Fire IV, the main party is forced to wage combat against a computerized security system, which consists of two giant metal boxes that cast spells.
 * A boss early on in Rayman 2 is a weak version of this. It spits out bombs with small helicopter blades on top that fly towards you (no, really). To defeat it, you have to grab a "keg" and throw it at one of three cartoony bandages on the side of the machine. Doing this will power down the electric cage which has trapped Ly.
 * Einhander has a mook like this. Twice during the second level, you have to face off a massive train car with respawning turrets and a control tower. To beat it, you have to destroy the control tower while being careful not to shoot down the radar on top, or else the tower will retract for a few seconds and come out with a new radar.
 * However, if you want to get two secret bonuses, you have to shoot down enough of the respawning turrets until you obtain it.
 * Castlevania: Legacy of Darkness had the "Security Crystal" which consists of a large, central crystal and several self-repairing turrets of various types placed around it.

Other
Fortunately, the third turret announces its timing, and you can learn a move that will reflect the move back at it...
 * In general, shmups made by Compile has this kind of boss. It's like a Compile trademark. In the earlier ones, like Zanac, mentioned above, it's the only type of boss you will face (not counting the mini-boss ships that appear once in a while). The more advanced the boss, the more cores and turrets it has, until they fill most of the screen. And the more cores and turrets you have destroyed, the fiercer the rate of fire of the remaining ones. Fun times.
 * Mega Man Zero 4 first boss is the turret system defending the squad sent to destroy the caravan Zero is protecting. There is a Pantheon merged into the core, but it doesn't move.
 * There's also the Mini Boss Tyrine from the same game, surrounded by Glass Cannons (no relation to Glass Cannon, they're pretty tough). Tyrine will shoot periodically while the Glass Cannons attack you, and if you destroy all the glass cannons, the main boss gets pissed. Destroying Tyrine ends the fight.
 * In the first game, the boss of the train mission is the engine, which also has a Pantheon merged to it.
 * Monster X from Cave Story.
 * In the Registered version of Raptor: Call of the Shadows, the Outer Regions set of levels features a number of non-Giant Ship bosses, all but one of them being heavily armed fortresses spraying generous helpings of plasma, heavy lasers, bullets and missiles at you. And yes, the Final Boss is also one.
 * Tales of Symphonia: The Defense system, guardian of Toize Valley mine.
 * Digital Devil Saga 2 has about three fights against security doors with turret duos with the last one havering a third turret that can put you in a world of hurt.
 * The Gradius series might as well be the Trope Codifier. "Destroy The Core!" is pretty much synonymous with the series and the Big Core is a major recurring boss series.
 * One boss, first seen in Salamander/Life Force, consists of a wall with three cores and half-a-dozen turrets and mook makers.
 * In some instances, the wall of cores and turrets you're fighting will charge at you.
 * Almost all of the bosses in the sixth-gen Star Wars: Rogue Squadron titles (Imperial Star Destroyers in both, two versions of the Death Star in 2 and the Executor in 3).
 * Portal:
 * Genetic Lifeform and Disk Operating System (G La DOS) is a textbook example of this trope.
 * In the sequel, Portal 2, G La DOS takes this a step further by assuming the position of Bad Boss of a modified Personality Core (Atlas) and Turret (P-Body), two robotic test subjects, making her the Core and Turret's Boss.
 * Inverted with in the sequel. Hitting the boss stuns it, allowing Chell to attach another core, in order to corrupt the primary core.
 * Resonance of Fate has one in the penultimate chapter. Lots of turrets, one core.
 * The VI, the final boss in the Mass Effect 2 DLC "Overlord".
 * Rock Man 4 Minus Infinity has the Serges Devil, which combines the Platform Battle with this trope.
 * For the final boss of Deus Ex Human Revolution, we have the
 * La-Mulana does this in a way with Viy, whose turrets take the shape of tentacles and can only be temporarily disabled.