I would say that the vocabulary used in this Croatian crossword is for beginner to intermediate learners.
What exactly is easy vocabulary anyway? Is it the numbers one, two, three (jedan, dva, tri), the words for cat and dog (mačka, pas) or something else entirely.
In truth, easy vocabulary is personal to the learner. I've been learning Croatian for a while and still haven't got around to learning the months, but know the word for society (društvo).
Regarding these crosswords, all I can say for definite is that you won't find a Croatian word such as antidisestablishmentarianism, and I've mainly used words which have come up in conversations that I have had, and more will be added as and when ...