I really like the setting of a real world war, but mixed with the steampunk elements like the machines and creatures
The contrast between Alek and Deryn is really nice. Alek being originally spoiled and having to adapt to the real world and learn that commoners aren't as bad as he thought + Deryn coming from a presumably humble (or at least poorer than Alek) background and achieving her dream only for it to be ruined is soooo good
I also like Dr Barlow's character. Her being in a weird position of being simultaneously looked up to due to being a revolutionary scientist and looked down upon for being a woman is so interesting and ties in with Deryn's character
I was legit on the edge of my seat when they met like I love how it was done
Also the secret package to Constantinople being revealed but not elaborated on is so good!! It keeps some of the mystery for part two while still being a satisfying reveal!!