ほいほい騎士団が誘い出された時点で、半ば勝負はついてるんだよ >>What does this mean?

Question

Context: A servant is talking with a prince, about an sealed egg that had been stolen from the cathedral. It can be resurrected to summon back an ancient being who can destroy the world. Meanwhile the Knight Order which protecting the Royal City is away from the city right now

S: 先日大聖堂より盗み出された例の卵が、復活しようとしているようです
(My Prince, it seems that the egg that was stolen from the cathedral the other day is about to be resurrected.)

P: ……あぁ、そう。そりゃこういうことになるよな
(...Oh, is that so? I see what's happening now)

P: ほいほい騎士団が誘い出された時点で、半ば勝負はついてるんだよ

I don't get this part. My guess is (When the Knights are being lured away, the game is half over /The enemy got 50% chance of victory already by doing that)

Please correct me and share your English TL if my understanding here is wrong

Answer 1

The first half of your translation should be right. I think you have a good understanding of that part of the sentence:

ほいほい騎士団が誘い出された時点で
By the time the knights were lured out,

In the second half of the sentence, 半ば is modifying 付いている, which is the 10th definition on jisho here. So, whether they win or lose (勝負) was already half decided.

ほいほい騎士団が誘い出された時点で、半ば勝負はついてるんだよ。
By the time the knights were lured out, it was already half decided who would win.

or, more naturally, "By the time the knights were lured out, the victor was already half decided."