Your sample is entirely correct and grammatically consistent as you wrote it. Past and present tenses can be used in the same sentence, as in
she misses the school that she left last year
they are unwilling to admit that they had been wrong about Bill
but in all such cases the earlier action (earlier in time as the action occurred) must be in the past tense and the later action in the present.
In general, two or more of past, present and future tenses can be used in the same sentence, but whatever be the tenses used, what happened earlier is expected to have an 'earlier tense' than what happens later. (Grammarians please excuse my terminology, as long as I am conveying the correct meaning!)
They insist that they have done their homework.
(Here 'insist' = simple present and 'have done' = present perfect which is the earlier tense of the two, since present perfect comes before simple present. )
She ran into the station but the train had left.
(Ran = simple past and 'had left' = past perfect which comes before simple past)
IN THIS CASE, 'six prisoners fled' occurs before 'he got the news' and so you need to say
"he got the news that (...) six persons had fled the castle.
Here got = simple past and 'had fled' = past perfect.
It also sounds just right as you wrote it and I would advise you not to change the current ('had fled') construction.
HOWEVER, because this is supposed to be fiction, which gives the writer much greater grammatical license, you can actually use the present perfect tense "have fled" as in "he got the news that the jail was under attack and six prisoners have fled the castle", but only if you are very particular, and understand that it is strictly not grammatically correct, and you are willing to knowingly make an error for the sake of getting your intended effect!
Some of the greatest authors have written many grammatically dubious sentences that sounded just right for the specific situation, and some have (unknowingly or knowingly) even made grammatical errors. AT LEAST IN FICTION YOU CAN BE THE MASTER OF YOUR OWN WORK!