Ridgeland Tower is in northwest Hyrule. This page explains how to get there and climb the tower.
Reaching Ridgeland Tower
From the top of the Great Plateau Tower, look northwest. You should be able to see three towers. Ridgeland Tower is the leftmost of the three. Put a pin on it with the Sheikah Scope.
To get there, float down off of the north wall of Great Plateau. There is a riding path that goes along the north wall of Great Plateau. On that road, go west. If you have not tamed a horse yet, tame a horse in the hills north of the Great Plateau. Then ride west along the road, and when the path splits, go to the right. You will reach Outskirt Stable. If you tamed a wild horse, you can register it there. If you already have a boarded horse, take it from the stable now.
Ride north from the stable. You will cross Manhala Bridge. When the path forks, go right.
Eventually, you will cross Jeddo Bridge. After that, go north when the path splits. You should be able to go toward the tower by sight at this point.
Climbing Ridgeland Tower
The tower is in a pool of water and is surrounded by Electric Lizalfos and Electric Wizzrobes. You can attempt to approach the tower without any of these enemies noticing you, but that might not be possible, so you should try to kill some of the enemies from a distance and then swim to the tower when that side of the tower is clear of enemies.