Sentinel Archives are archives created by Sentinels which logs the details and records of their findings during their field expeditions outside the safety of the wall.
Sentinel Archives can be found in and around Sentinel Camps. These camps are the places where the Sentinels are keeping a watch and protecting the civilians outside the walls, or keeping a watch for dangerous activities. These archives show up as 
icon to the Freelancer. Different number of such archives need to be collected so that they can be decrypted into the cortex entry. Usually, collecting 5 such collectible notes would suffice to obtain one entry.
Archives are obtained by collecting these collectible notes around Sentinel Camps or patrol areas. Other Sentinel Archives are also available all around Northern Bastion. The ones from Sentinel Camps are needed to complete the Challenge called Sentinel Archives Challenge and Master Archivist challenge. Both of these are required to obtain the No Stone Unturned Achievement.
Collection Details
A total of 5 cortex entries make reference to Sentinel Archives.
Several Sentinels have logged their perspectives on the people of Coda through these Archives. These can be found as letters, documented complaints, advertising pamphlets, etc.
Sentinel Archives (1)
See Cortex Entry: Sentinel Archives (1) for More Details
The Emperor wants you!
The Sentinels are looking for motivated young people to join their ranks. Lancers, cyphers, and Arcanists are all welcome to apply for various positions. We have high expectations for our recruits. Applicants must possess basic education and a strong connection to the gateway. Good moral character and an eagerness to protect the free people of Bastion are required.
Follow Vassa's way and stand tall as you keep the citizens of Bastion safe Join the Sentinels!
Sentinel Archives (2)
See Cortex Entry: Sentinel Archives (2) for More Details
Tracking number: 3476.
Name of Complainant: Pirndel Blatch.
Address: #18, North Wall. Fort Tarsis
Complaint: I think my neighbor is a Regulator
Comments: I've noticed strange comings and goings at my neighbor's place over the past few weeks. No activity during the day, but then LOTS of activity with shady characters a few nights a week. There's a strange smell coming from their apartment (#19 North Wall) that I think is stinkhand. As you know, stinkhand is banned from Fort Tarsis because of its awful smell, and who would have access to that except Regulators? Thank you for your haste in this matter.
Sentinel Archives (3)
See Cortex Entry: Sentinel Archives (3) for More Details
They looked carefully over the wall's edge. "Look at that Freelancer… blasting off into the great unknown. What good would we be if we left our post to traipse around the wilderness?"
Calahan looked at her. "Tarsis died to protect the people of Antium. If you're going to kick the bucket, it should be following her example. Not while off on a whim."
Sentinel Archives (4)
See Cortex Entry: Sentinel Archives (4) for More Details
Vassa remembered the General's words when she smote the urgoth at the pass. That Antium, and all other settlements, must be protected even at the cost of her own life. And Vassa did say, "Go forth and know that as you stand sentinel at our walls, your vigilance and bravery bring peace to those who cannot pass the gateway."
— From The Volume of Tarsis
Sentinel Archives (5)
See Cortex Entry: Sentinel Archives (5) for More Details
Proclamation from Antium:
It is declared that a squadron of Sentinels will be transferred to Fort Tarsis to better protect the people and paths of Northern Bastion. Please see to preparation of the proper quarters.
Regards,
Imperator Marjan, Antium
In the name of the Emperor of Antium, Valus Dell.