(Sandstone Watchtower isn't a spell.)

At the beginning of your end step, if you didn't cast any spells this turn, Sandstone Watchtower deals 1 damage to you.

Sacrifice a land: You gain 1 life.

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