There is practically no other choice on a Z80. C compilers really don't like the Z80 ISA and create both slow and bloated output, at least compared to manually written asm. And on top, you can do many optimization tricks in assembly that simply are not possible in a high level language (self-modifying code, jumping into the middle of an instruction, etc...).

Forth might work better than C, but tbh, assembly code is more straightforward and easier to read than Forth ;)