If you have reached the Cursed Swamp and the Stargazer Tower POI then you are likely as stumped as we were by this puzzle. The answer is actually embarrassingly obvious but at least we can share in the shame together!
Windrose Stargazer Tower Puzzle Solution
Once you reach one of the Stargazer Towers, which are basically small temples/ziggurats, you will see that the stairs at the front lead down to a locked door with two stone plates on either side. Both plates can be rotated into one of four positions.
We spent way too long wondering where the answer to this puzzle was, running around to the top of the temple, checking the sides and trying a bunch of other logical configurations. However the answer is painfully simple, all you need to do is tilt your camera up above the plates.


The above image shows the correct positions for each plate and the panel above. The left image shows the left plate, and right image shows the right plate.
Above each plate there is a panel which shows you which one of the four patterns needs to be at the top on each side. Simply rotate them until the top pattern matches the plate above and the door will open. Told you it was embarassing!
Windrose Stargazer Tower Rewards
Each of the Stargazer Towers also contain an Ancient Chest that contains 3x Tumbaga Ingots and an assortment of other materials, looting this Chest will also 'Complete the POI' and award you with your XP.
Fate Of The Prophets Side Quest
After completing one of the Stargazer Tower puzzles, you will see an 'Observatory Stele' directly ahead. Interact with it to start the side quest: Fate of the Prophets. This quest rewards 10x Tumbaga Ingots (huge!) for a total of 19 Tumbaga Ingots as a result of completing this quest chain.

To complete this quest you will be sent to another two Stargazer Towers, and required to complete another two puzzles which are the same as the first. The Stargazer Towers are all heavily defended with a horde or Level 15 Plague enemies so come prepared for some tough combat.
After you complete all three Stargazer Towers you have to find the 'warlord's death site' in the centre of the island. When you reach the location you'll see a named Plague Crusher called Chick Chan wandering around in the swamp, this is your target.
There's not too much to say about this fight as the Crusher doesn't have a tonne of abilities, just use ranged attacks to burst the 'boils' and break Chick Chan's posture then get plenty of attacks in while it's down. It has a tonne of HP so we recommend coming with Level 15 weapons and a bunch of Healing Potions.