Many subsequent compilers were written in an assembly language as well. But the first C compiler was a modified B compiler, which was written in B. The first B compiler was written in TMG. The TMG compiler used to compile that B compiler was written in PDP-7 assembly language.

Do compilers compile to assembly?

GCC compiles to assembler. Some other compilers don't. For example, LLVM-GCC compiles to LLVM-assembly or LLVM-bytecode, which is then compiled to machine code.

What are compilers written in?

Today, the first compiler for a new language is often written in C, but when the language reaches a certain maturity it is often rewritten "in itself". The first Java compiler was written in C, but later rewritten in Java.