Swamped

"How else would I have convinced him to let me escape?"

It was an exchange of information. Your goal was to gather evidence about the poison and Long delivered. The threat of an attack was the one card you had to play. You used it to get your info and suggested you could avert an attack if he released you. You'd return to base with information in hand, mission success.

You didn't know that the attack had been approved, nor that they'd send half of a weakened force out while a group of Bogknights crept towards the fortress. Not to mention they ordered the attack without waiting for you to return with actual information.