Conan. I had to mention him again after he got mentioned in both of my previous alphabet entries. I don't have a whole lot more to say about him that I haven't already covered. He's a huge influence, etc. Read the Howard stories, read the comics and watch the Schwarzenegger movie; that stuff will bleed D&D inspiration all over you.
C is also for Celts. The Celts dominated Europe before the rise of Rome. Seriously, Celtic culture was everywhere from the western edge of the Black Sea to the Atlantic ocean, from the northern shore of the Mediterranean to the North Sea and the Baltic. All of central and western Europe were Celtic territory. Northern Italy, France, Spain, the Czech republic, Germany, Austria, Switzerland, the low countries and all of the British Isles. They are more of an ancestral "lost" civilization than the Romans are to anyone of central or western European descent. They are probably the first Europeans to work iron. They were conquerors and artisans. They beat the Greeks and sacked their holy sites, including Delphi where Apollo's oracle was, after which they planted a colony in what is now central Turkey; they were the Galatians of the bible. They beat the Romans and extracted tribute from them to quit their occupation of Rome. Seriously, that we are not communicating right now in a Celtic language is a result of them peaking too early and lacking in centralized government. Also, they scared the Romans. That caused the Romans to need to extinguish the Celtic culture as nearly as they could.
C is for D&D's Common Tongue, a language I have long had a love-hate relationship with. I can almost think of Common as simply the lingua franca of the D&D world, like English is today in our world, or French was for centuries previous to it, or Latin before that. The problem then with Common as a language is that there should still be local languages and dialects. If that is the case though, then the characters are going to spend all of their language slots on other human tongues rather than the myriad of non-human languages they need to communicate with the denizens of the dungeons. In my campaign I use a variety of "national" languages anyway and I have a common tongue used by educated people and traders. Shouldn't all of the other races have a wide variety of languages too? Do all Orcs really speak standard Orcish? All Elves Elven? Dwarves Dwarvish?
C is for Clerics, who I recently did a blog post on, so I won't cross over too much of the same ground here. I'll just say that the "standard" Cleric of D&D is not really how I envisioned the Garnian priesthoods, I see them as descended from ancient Druids; not D&D Druids though. The Garnian priesthoods are more like the 2nd edition AD&D Specialty Priests. That said, a number of fighting orders in Garnia are practically indistinguishable from standard AD&D Clerics, and they are where pretty much all of the adventuring priests come from in my campaign. I guess it's my justification for them that makes the difference to me.
C is for Claymore, which is an Anglicization of the Gaelic claidheamh mòr; which just means great sword. Claymore actually can refer to either of two types of Scottish sword. The first is a pretty distinctive two handed sword notable for it's angled cross-hilt guard. The second is a much later development, a basket hilted broad sword. Both are used primarily by Scottish highlanders and the second is thought by some scholars to share the name of the first simply because it was common to cut down the bigger swords into the smaller basket hilted version when great swords fell out of fashion. The Claymore was used as a sidearm in battle as late as world war 2 and is still the dress sword of Scottish Highland regiments throughout the British commonwealth. The Claymore had such a bad-ass reputation that the US Army named the M18a1 anti-personnel mine after it. I like either variety of Claymore because of my Scottish Highlander ancestry, their usefulness as weapons and their distinctive look. If I had to pick a favorite I would go with the Basket hilt because I can still use a shield and my sword hand is much more protected. I have fought with both in the SCA.
C is for Communism, the bogeyman of my youth. They got to be the villains in movies (particularly Red Dawn) and games, both RPGs (Twilight 2000) and board games (Fortress America, The Next War). As an American they were the enemy. The Soviets, the Red Chinese, North Korea and Viet Nam were enough of a scare to cause us to generate deficit spending for the first time in US history in order to win the Cold War. After the Soviet collapse I started looking into what all this commie stuff was all about and realized it wasn't all that bad. Totalitarianism was the real problem, not the economic policy of socialism leading towards communism; but, hey, I'm an intellectual and everyone knows we're all Commies, right?
C is for Combat and man is that a big part of D&D. Probably bigger for me than it is for a lot of old schoolers because I am a combat junkie. I usually fight rather than negotiate even to my own detriment. I must just have a bellicose nature.
C is for Caves. Since the beginning of my D&D playing life caves have been where we went to find the bad guys, kill them and take their stuff. From the Caves of Chaos to the Lost Caverns of Tsojcanth. It's a winning formula and if it ain't broke, don't fix it.