Arcane Observatory

Kael'thas was ordered by Garithos to fix the Arcane Observatories near the retaken city of Dalaran. The first was being repaired when Kael'thas arrived. He fought against Blood Wizards and other enemies to reach the second. The second was on an isle. And the third was on an island protected by Snarlmane the Bloodgorger. After Kael fixed them Garithos took away his human and dwarven forces leaving just the blood elf forces. Then, the Dreadlord Dalvengyr attacked with his forces of Undead. The Naga sea witch Vashj helped Kael'thas destroy Dalvengyr.

Notes and trivia

 * They have a huge vision range
 * They look a lot like the Violet Citadel, more so than the citadel's model.

Speculation
The second observatory may have been in the Fenris Isle archipelago.