Unicorn vs Nightmares

Do you know where nightmares come from?
They come on the nights the unicorns are not watching over you.

Tonight, you are the one who watches. Place your unicorns across the dream and
link them back to your castle. Collect the stars scattered along the way, and
cover the monster's tiles to steal the ones it is sitting on. Spend the powers
on your cards: draw more, play again, sweep an enemy off the board, or send in
the little pink one that beats absolutely everything.

Reach the monster's castle, or hold your stars through one whole turn of its
attacks, and the nightmare turns back into a dream. Then the next dream opens,
a little darker than the last one.

How long can you keep the nightmares away?