Nithilar, Canyon Keeper

Defender, Firststrike
Opponents attacking you or a Plainswalker you control may only attack you with one creature.
Sacrifice a Forest, Nithilar, Canyon Keeper may attack as though she did not have defender.
The only way to attack Witherwood from the South is to pass over Yasurath Bridge. Nithilar guards Yasurath Bridge, therefore you cannot attack Witherwood from the South.