Saka is not an adjective but a noun that means slope.
e.g. The road is hilly.
e.g. Nagasaki's hills are tall but not steep.
I hasten to explain why I have okayed saka
as the translation for "hilly" in the first sentence despite my statement that saka
is a noun. Some Japanese nouns can be used adjectivally. But this is a derived
function and not always applicable. I am not sure if there is a clear-cut rule which nouns are adjectivasable and which are not.
Even in the first sentence, saka
loses its adjectival force when deprived of the adverb ちょっと.
In the second sentence, I cannot interpret 坂ではありません as a negated adjective, "is not hilly." Instead, the most usual understanding of the sentence would be, "Nagasaki's hills are tall but not slops."
I would suggest rewriting the translation as;
A bonus pack.
Kyūda can be attached to saka
too; 急な坂. The opposite state is ゆるやかな/ゆるい坂.