About 1739 years before the battle of Geonosis was a time of great war between The Army of the Sith, The Tyrany of Mercenary's and Scum of the like, And the Republic army any the Jedi Order. Each side had its own agenda in this Great war. The Jedi wanted to do what they have always wanted to Keep order and peace throughout the Galaxy. While the Republic wanted to keep itself stable and not lose planets from the Great Republic. The Mercenary's Only Desired credits and loot of the like some had other agenda's in this war like getting back at some higher ups in the Republic or even the want to just kill without law stopping them. The Siths agenda was too do what they always have and will always want to do Control the Galaxy with a iron fist, and practise their dark powers on powerless civilians.
Here is a Concept of what the Sith Architecture might look like on Cossintan.