CPCKey is now several years old and I learned a lot since building it. I still use mine, but I would not construct it the way it is built again. These days I would just use a single chip flash micro-controller and do the whole keyboard matrix handling in software. I will leave the project online in its current state, but I don't think it is still useful to build a CPCKey.
With CPCKey it is possible to connect a MF-2 keyboard to a computer of the Amstrad CPC series. It contains a small Z80 system to translate the MF-2 keyboard codes to the CPC keyboard matrix. Its main purpose is to solve the keyboard problem if you want to put your CPC into a bigger case.