Tiny Castle

Creates a level 3 Town Hall at a target location.

Source
Kael can find one in Misconceptions, chapter 1 of the Frozen Throne alliance campaign, Curse of the Blood Elves. If you do not use it in this chapter, it will be gone from your inventory in the next chapter.

Kael and Lady Vashj can also find one in A Dark Covenant, chapter 2 of that campaign. If you do not use it in this chapter, it will be gone from your inventory in the next chapter.