Caer Oswin is the HQ of the Seekers. You visit it for the first time while completing the Inner Circle quest for Cassandra- "Promise of Destruction". It is another quest that you will be able to complete for that party member so, you do not receive it in the first place. The castle is a simple location with no items and no collectibles. The only thing here is the numerous hostile Seekers.
The way to unlock the location: Keep completing quests for Cassandra (Inner Circle quests, available after you reach Skyhold), up until you receive "Promise of Destruction". Use the war map, then, to travel to Caer Oswen.
Recommended experience level: You can start the quest only after you reach Skyhold, at which point your experience level will be sufficiently high. The recommended level is 13-15, but due to the low number of enemies, you could start it at level 10.
1) You will not be able to return here so, collect all the loot on a regular basis.
2) Cassandra is your compulsory companion, during this quest. If she is not a regular member of your party, equip her with armor and a weapon.
3) In spite of the low level of the majority of opponents here, towards the end, you are going to fight Seeker Lucius, who may prove a challenging opponent so, be prepared for the battle.