Your secret Hideout can be found by completing the quest The Old Tale (Parts 1 and 2). It can be used to house some of your soldiers - a maximum of 50 once you get it, but this can be increased to 100 - and also hire some of your own merchants and workers. The troops stored will have 75% of their normal wage, and the permanent crew's upkeep will be 500 denars on top of that (who cannot be disbanded).
You also can rest here for free, and have a completely safe Chest you can use as well (ask Bernard to open the inventory). This site originally is known as Old Ruins on the map, and is located just west of the village of Kwynn inside a mountain cove.
Your Camp Master is Bernard, who you hire as part of The Old Tale. He gives you access to the camp's inventory, he can hire additional personnel, and also can construct additions to increase the size and usefulness of the camp in general. To increase the size of the garrison costs 15000 denars, and this can only be done once.
Your hireable Blacksmith for 3000 denars is Harold, who not only gives you the Barding quest, but also can repair poor quality and damaged items up to their normal values. Unlike Alexios of Melitine in the Snake Cult Stronghold however, he cannot make the items better than their normal value.
You also can hire a stableboy for 5000 denars, a Jatu named Derafsh, who can heal poor-quality horses up to their normal values. Once you pay for him, you also have to go out and hire a Jatu Outcast Rider and bring him back to your Hideout (the idea being that guy is now Derafsh).