Complex sentence: This could be due to the general nature of the prisoners that were sent there; most of them had attempted escape previously from other prisons and were transferred to Colditz because the Germans had thought it to be escape-proof.
Split into simpler sentences: This could be due to the general nature of the prisoners that were sent there. Most of them had attempted escape previously from other prisons and were transferred to Colditz because the Germans had thought Colditz to be escape-proof.