Each Great Rune is tied to a Divine Tower, and to unlock the full potential of these boss items they must be activated at their corresponding Tower. There are six in Elden Ring spread out throughout the Lands Between, though there are some that can't be accessed directly.
The Divine Tower of Limgrave is tied to the Great Rune of Godrick, and is connected to Stormveil Castle itself. When equipped, the Great Rune of Godrick increases all stats by five points, a massive boon in early levels and a helpful boost to all-around performance later in the game.
To get to the Divine Tower of Limgrave, head directly into the front gate of Stormveil Castle and make your way through the ballistas and soldiers to find the Limgrave Tower Bridge Site of Lost Grace towards the back of the keep. Cross the bridge, take the Waygate teleporter to the other side, and get past several Golems to reach the top.
The Divine Tower of Liurnia is tied to Ranni's questline, though one wouldn't be faulted for thinking it's tied to Rennala's Great Rune. You can only access this location after progressing Ranni's questline significantly, and the reward is tied to unlocking new endings for the game rather than a new Great Rune ability.
To get to the Divine Tower of Liurnia, use the Carian Inverted Statue at the Carian Study Hall in the center-east of Liurnia (it juts out from the side of the area). This item is given by Ranni after unlocking Nokron and finding the Fingerslayer Blade, though it's a simple task of ascending the inverted Study Hall and getting past a Godskin Noble after that.
The Divine Tower of Caelid is tied to the Great Rune of Radahn and features a difficult jumping puzzle segment to get into in the first place. The Great Rune of Radahn increases HP, FP, and Stamina by 15% – a massive boost, though it is limited to resource improvements rather than stats.
To get to the Divine Tower of Caelid, head to the northern area of the Dragonbarrow and look for a cliff leading to the gigantic Divine Tower just beyond the edge of the area. Carefully navigate around the tower until you find the Site of Grace before the elevator. There are a lot of good items spread around this area aside from unlocking the power of the Great Rune, so do be sure to explore the lower levels and outdoor areas surrounding this Divine Tower.
The Divine Tower of West Altus is tied to the Great Rune of Rykard and has an entrance more secret than any other Divine Tower in Elden Ring. The Great Rune of Rykard restores HP for every enemy killed, just like the special effect of the Blasphemous Sword without using his Remembrance. It's not especially useful for bosses, but very handy in the open world.
To get to the Divine Tower of West Altus, head to the Sealed Tunnel in the south of the Altus Plateau. You can see it easily on the world map, even if you haven't discovered it, as there's a reddish-orange cave icon noting its location. There are a number of Illusory Walls in this dungeon - behind one of them (and a Onyx Lord boss) is a teleporter that takes you directly outside this Divine Tower.
The Divine Tower of East Altus is the only Divine Tower tied to two runes - the Great Rune of Morgott and the Great Rune of Mohg. Both will appear if you have both in your inventory. Mohg's Great Rune lets invaders heal from kills and increases damage after causing Hemorrhaging. Morgott's Great Rune is simpler and increases Max HP by 25%.
To get to the Divine Tower of East Altus, you need to have access to the Great Lift of Rold and the Forbidden Lands. Instead of taking the elevator, exit the Great Lift of Rold area to the south, which leads to a boss fight against the Fell Twins shrouded in darkness. Defeat this duo and you will be teleported to the foot of the Divine Tower of East Altus.
The Isolated Divine Tower is tied to the Great Rune of Malenia, and thus is typically the last Divine Tower players visit in Elden Ring. Malenia's Great Rune makes attacks the player makes heal a small amount of HP after they have been damaged. Basically, if you get hit and quickly deal damage back, you'll recover a good portion of HP.
To get to the Isolated Divine Tower, use the Teleporter Trap in the Tower of Return, located in the south of the Weeping Peninsula beyond the Walking Mausoleum. Activate the Site of Lost Grace, then head to the Fortified Manor in Leyndell to take the lift up to the same area and activate a Waygate just past a Golem. This Waygate teleports the player to the Isolated Divine Tower.
Elden Ring is available now for PC, PlayStation 4 and 5, and Xbox One and Series X|S.