Classic LEGO Castle

The original LEGO castle theme. Along with LEGO Space and LEGO City, this is by far the longest-running of all LEGO Themes.

The earliest sets appeared in 1978-1979 and were among the first to feature more specialized building parts outside of the usual bricks and minifigures (new items included simple-shaped medieval helmets and melee weapons like halberds, lances and shields). Sets from the first half of the 1980s started to get more sophisticated and added a lot more unique parts including swords and - notably - purpuse-built horse models (despite a small esthetic overhaul in the 1990s, the basic template for LEGO horses hasn't really changed all that much for 30 years).

It received an odd Spin-Off in the early 2000s, titled Knights Kingdom 2 - even though it had little to do with the Knight's Kingdom subtheme.

Eras and subthemes (often based on a respective faction) in chronological order :

 * LEGOLAND Castle or just Castle (1978-1986) - the original showcased a generic medieval kingdom with sets revolving around generic medieval locations and events (e.g. castles, jousts, men-at-arms escorting wagons); as evidenced by the title, it is often distinguished from the subthemes that came after it by the "LEGOLAND" prefix (named after a common feature of LEGO box art from the 1980s and early 1990s)
 * Crusaders (1987-1992) - the first somewhat more specific theme that evolved from the original, though still lacking a distinct faction with a distinct backstory or motivation
 * The Forestmen (1987-1990) - the first outlaw subtheme, with Loveable Rogue bandits in the style of Robin Hood
 * Black Knights (1987-1992) - the longest-running of the "faction specific" subthemes, notable for showing a kingdom ruled by "The Black Monarch" and The Teutonic Knights-like Order of Black Knights, featuring a lot of cultural elements from the Baltic, north German and Scandinavian Middle Ages (LEGO being a Danish company, this is probably intentional)
 * Black Falcons (1990) - an oddity, consisting of only one known set; they were probably meant to be a part of the Black Knights subtheme, though the title suggests they may have something to do with lord Black Falcon, who appeared in the first era of the Castle theme
 * Wolfpack Renegades (1992) - the second, more grittier and morally grey outlaw subtheme, their heraldic figure was a silver wolf head
 * Dragon Masters (1993-1995) - a group of dragon-themed Magic Knights (complete with dragon-themed horse armour), who had assistance from the Merlin-like wizard Majisto and several tamed dragons (this subtheme is notable for introducing the wizard minifig accesories, unique horse armour and the iconic LEGO dragon figure with detachable red wings); the Knights heraldic figure was (of course) a green dragon with red wings
 * Royal Knights (1995-1997) - the introduction of King Leo and his brave and gentlemanly Royal Knights, a very straight example of The Kingdom; their heraldic figure was a roaring lion head with a crown
 * Fright Knights (1997-1998) - a more campier line with a Dark Is Not Evil flavour : witch minifigs, black dragons, spookier castles and the titular successors to the Dragon Masters, who had a prominent bat motif going on (from armour and helmets to various tiny details on their architecture), with a bat being their obvious heraldic figure
 * Ninja (1998-1999) - Spin-Off with a Jidai Geki / Wutai setting
 * Knights' Kingdom (1999-2000) - best described as a Revival of the Royal Knights theme with some updated esthetics, King Leo gained a formiddable new enemy in the cunning warlord Cedric Bull, who led a rebel army with higher than usual morale and an excellent arsenal of various siege engines; Bull's heraldic figure was a red-eyed black bull head; this subtheme also received a (pretty much forgotten) video game adaptation
 * KnightsKingdom2 (2004-2006) - a major departure from the usual formula, trying to ape the success of the more action figure oriented LEGO themes of the time, (i. e. Bionicle); often regarded as The Scrappy or even a borderline Franchise Killer
 * Castle (2007-2009) - intended as a return to the series typical style and as a Continuity Reboot of sorts, with a Captain Ersatz version of King Leo defending his kingdom from the undead armies of a dark ruler (the undead are represented by skeleton minifigs)

Tropes used in this LEGO theme include:

 * Animal Motifs : The various heraldic creatures of the different factions. Wolves, lions, bulls, dragons, bats, you name it. A subversion were the Black Knights, who used a blue-coloured wyvern in their coat of arms, despite their name.
 * The Archer: Many of the soldier minifigs. There were even sets with ballistae and other crossbow-derived siege engines, complete with operators.
 * Art Evolution : More unique and specialized items, accesories and minifigs (horses, ghosts, skeletons, witches, wizards, dragons, medieval weaponry and armour) got gradually introduced as the years went on. A good example of this is the case of the horses, which overlaps with Early Installment Weirdness : The horses from the late 1970s sets were still abstract constructs made from LEGO bricks until proper horse minifigs were introduced in the early 1980s.
 * BFS : Lord characters always wielded bigger and shinier swords than the regular soldiers.
 * Cool Horse : Since the 1990s, the various factions often had unique horse armour at their disposal.
 * Cool Ship : The Black Knights were the only faction to prefer coastal or river enviroments and own quite a lot of ships. The biggest ship set of the castle theme, the war cog "Sea Serpent", fully fits this trope.
 * The Engineer : Cedric Bull's faction sure loves building various siege engines...
 * Dark Is Not Evil : The Black Knights and Fright Knights.
 * Fantasy Gun Control : Played almost completely straight. Several of the Ninja sets had early Japanese firearms and one of Cedric Bull's siege engines looked and worked awfully like a cannon.
 * Feudal Overlord : The rebellious Cedric Bull, who also doubles as a Dangerous Deserter.
 * Gang of Hats : All the factions, some by atittudes, some by their preferences for certain types of weaponry, some by the use of magic, etc.
 * Instant Awesome, Just Add Dragons: The Dragon Masters and Fright Knights lines.
 * Instant Awesome, Just Add Ninja: Well, duh, the Ninja line. However, despite the title, a lot of the sets from this line also contains Samurai-themed sets and minifigs. The line could be more accurately called "LEGO does feudal Japan", but Ninja was apparently a shorter title... and, you know, more awesome sounding.
 * Legacy Character : Arguably King Leo, who's the only character to appear in three different subthemes, years apart (though in the third and latest one, it's actually more of an Captain Ersatz of him).
 * Jidai Geki : While the European-themed sets are grounded in a realistic Medieval European Fantasy world, the Ninja sets play this trope for all its worth.
 * Katanas Are Just Better : They feature prominently in the Ninja sets, but this gets subverted by the presence of various other medieval Japanese weaponry. Besides, the swords look "generic Japanese" enough to possibly represent various other actual sword types similar to katanas, but not the same as them.
 * Long Runner : One of the three longest-running LEGO themes. Also, a lot of the still-being-used medieval and creature models.
 * Loveable Rogue : The Forestmen. The Wolfpack were a more sinister bunch of outlaws.
 * Medieval European Fantasy : Though some of the factions have a certain degree of Fantasy Counterpart Culture to them, they are mostly "generic medieval". E.g. King Leo's Royal Knights and the Forestmen play this straight (generic western European kingdom and generic Robin Hood-style outlaws), but the Black Knights are a more concrete analogue of a real historical group (Baltic/German/Scandinavian-style nobles and knights with an affinity for sailships and coastal castles).
 * Point Defenseless : Awesomely averted by King Leo's castles, which include defensive catapults to counter an enemy's barrage.
 * Shout-Out : Probably completely unintentional, but the Black Knights and Dragon Masters wore armour that often resembled the Knights of Ni.
 * Siege Engines : Featured as early as the second and third generation (Crusaders and Black Knights), but Cedric the Bull from the Knight's Kingdom era took this Up to Eleven (to the point of it being his gang's hat). The Dragon Master's also had a large trebuchet-like catapult with stylized dragon decorations.
 * Spin-Off : First the Ninja theme (regarded as the main theme's Asian cousin), then Knights Kingdom 2.
 * Ye Goode Olde Days: Granted, this is Medieval European Fantasy for kids, so despite some obligatory level of grittiness, don't expect any Darker and Edgier hyper-realistic portrayals of the Middle Ages.
 * Word Salad Title : One of the sets from the Crusaders subtheme era was called "Dungeon Hunters". Furthermore, a small Royal Knights set showcasing King Leo and his personal weaponry was titled "Royal King".