This is your ultimate goal. You have to solve all of the previous puzzles, first.
I have solved
I have two signet rings
So, you have solved the sundial and the bookshelf?
Then you should have found the two signet rings. One inside the bookshelf and the other going down the sundial stairs and taking it from the prisoner.
Okay. Then, open the gate.
Go through the guard towers. Henrik from the other and Tove from the other. The signet rings must be inserted into the gate-opening mechanism the same time. The gate will open.