now this is my first time doing a review of this scale so there might be some "oddities"...
it's (mainly) a parody of Fate/Stay Night, Fate/Stay Night: Unlimited Blade Works, Fate/Hollow Ataraxia, Tsukihime, and Melty Blood and you must at least see/read Fate/Stay Night and Tsukihime anime/visual novel to have any idea what is going on. Now I mainly saw the Fate series and have some limited understanding about the Tsukihime ones so this review will be mainly be on the Fate series (although to be honest the Tsukihime segments didn't really follow the source material much to begin with, so I was able to get away with it).
There's 3 seasons with 4 episodes in each, episodes usually alternate between Fate and Tsukihime segments (although there are some episodes that shares half and half).
Overall each episode is quite entertaining and doesn't have to be watched in order (with the possible exception of episode 11) as they're all self contained.
The entire Holy Grail War has been reduced to series of wacky competitions and all the characters are on good terms with each other, Lancer (from Fate/stay night) is the victim of some pretty hilarious jokes throughout the entire series. Arcueid and Ciel are now fighting over Shiki quite aggressively for various reasons.
the animation quality is quite good and lively, all the characters are drawn in a more "rounder" feel and was more "cuter" looking than before, environments are diverse and the colors are quite lively in feel.
overall the animation is more smoother and has less dark tones compared Type-Moon's previous anime adaptions.
As the name of the anime implies, it's a carnival it's full of lively music and sounds and there's never a dull moment. Need I say more?
the characters are quite amusing and one must seen/read the source material because by themselves they are very confusing and tend to be totally opposites/extreme versions of their normal selves
the series as a whole is quite amusing and TYPE-MOON fans should definitely watch it, but be warned you must at least seen/read Fate/Stay Night, Tsukihime, Melty Blood and Fate/Hollow Ataraxia to understand the contents.
update: since the previously uploaded video is no longer available watch the creditless opening instead