(Goblin Watchtower isn't a spell.)

Goblin Watchtower enters the battlefield tapped unless you control an enchantment.

Goblin Watchtower is indestructible. (Destroy effects and lethal damage don't destroy it.)

, Pay 1 life, Sacrifice Goblin Watchtower: Search your library for a Mountain or Plains card and put it onto the battlefield, then shuffle your library.