Garden of Avalon, also called GoA is a series of short story focused on TM's version of King Arthur mythology. Personally I consider it one of the more interesting works Nasu had wrote, the story is told through Merlin's flash back from the moment Arturia pulls Caliburn to her eventual death after FSN.
GoA is written as a way to re-conciliate the differences between FSN, FHA, and FZ, since Gen's FZ managed to attract a lot of hate on Arturia and conflicted with plot points in FSN. As a result Nasu actually had to move his ass to release GoA to fix this issue, clarifying once
and for all, TM's version of King Arthur.
Overall, GoA is a series of pretty well written short stories that function as a bedrock for later works of Fate franchise, especially post-FGO age, as a lot of the plot points and story elements of GoA boomerangs back into focus.
Overall, if one wish to get deep into the fate universe GoA and clock tower 2016 are some very important short stories that need to be checked out. As they both flesh out characters with a small but well written narrative.