Ivory Tower

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Creates a Scout Tower at a target location.

In team games, a non-human player may buy them from an allied Arcane Vault and then upgrade it to an Arcane Tower since it has no requirements.

Source
These can be bought from Arcane Vaults in Frozen Throne.

Kael can find two in Misconceptions, chapter 1 of the Frozen Throne alliance campaign, Curse of the Blood Elves.

In the RPG
In its portable form, an ivory tower is a 6-inch-tall model of a guard tower carved from the tusks of a kodo beast.