Hello, Arka, and welcome to the Grammar Exchange.
"The steep spiral staircase up to the top of the tower seemed the easiest thing in the world after that."
As far as i understand, we must not omit the "to be" after seemed because it is followed by an object compliment. Correct me if i am wrong, thank you!
Actually, the sentence is fine without "to be" after "seemed" ("to be" is optional there) and does not contain an object complement. Nothing is wrong.