The fortress is in Satorl Marsh on east of the Dark Swamp, south-east of the Silent Obelisk, west of the Glowing Obelisk and north of the Igna Territory (shortest way is from Igna).
The altar is on the roof, to get there you you need the secret door, what is behind the throne. If you have completed the The Giants' Treasure quest then next time near to the altar is lvl 91 Eternal Palsadia who hear if you run or walk.