It's a K -sound. The letter K wasn't used much by the romans at that time. They borrowed it from the greek alfabet (Kappa). Everytime you see a word in the romanic languages that has a K in it, it means it's a word with foreign origins.
Ah! Connected to German 'kaiser' then, probably.Thanks
Yes - also the origin of the word Tsar (or Czar).
Got me curious. Oh Wikipedia, what won't you answer?