/recurse:[dir\]file
where:
The /recurse option lets you compile source code files in all child directories of either the specified directory (dir) or of the project directory.
You can use wildcards in a file name to compile all matching files in the project directory without using /recurse.
Compiles all C# files in the current directory:
csc *.cs
Compiles all of the C# files in the dir1\dir2 directory and any directories below it and generates dir2.dll:
csc /target:library /out:dir2.dll /recurse: dir1\dir2\*.cs