I have, for a long time, been looking for a novel I read in my youth.
It was a pocket, and written for young readers 8-12?
I only remember fractions, in chronology, space travel (thoroughly and vividly described very colorful). I believe one traveller was lost in space.
The hero was captured, but manages to escape the back door of a moving vehicle. Only to be captured by birds and carried to nests in high trees.
Once out of that situation, in the city the people are drugged and in a zombie-like state. Somehow the solution is to sneak in to the large fort to save the day!