This place was built by Tsar Nicholas I after the November Uprising of 1830. The 10th pavilion, which is open to public and which you can now visit, was a place where political prisoners were kept. Cytadela was also a place where many of them were killed later on for their revolt against the Russian repression. More about the citadel can be read here.
I suggest you take some time to walk about, read the names on the board in the Gate of Killings and try to understand the overall atmosphere. The open part is now the X th pavilion with an exhibition in the Museum of Independence. Also, you can see paintings of Aleksander Sochaczewski, a Pole repressed in Siberia since 1863.
I always come here on a foggy autumn or winter morning when it’s cold or windy or rainy, when you can expect no one else there. Because this place is huge and there are not many watchmen, you can feel as if you were not just visiting. This is probably a huge oversimplification but I just don’t feel like visiting this place on a sunny afternoon would be appropriate.