Feather and spells turning permanents in to Creatures
No, it will not.

Feather, the Redeemed triggers on casting a spell, which happens well before the spell would resolve and have any effect such as turning its target into a creature.

When you cast Siege of Towers, it is not targeting a creature you control; it is targeting a land you control. By the time the mountain becomes a creature, it is well past the chance for Feather, the Redeemed to trigger. Unless you are targeting a land that is also a creature.