You have a higher chance to get the bonuses for having the correct lands with the duels. Also Zoo needs to win really early so you can afford the life because if you don't win in the first 5-6 turns you lose.
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I highly recommend changing out -4 Essence Warden for +4 Wellwisher. Also i would put in +3Elvish Archdruid and -3 Arbor Elfs
Thanks Broski
Overgrown Battlement would be better then Axebane Guardian, cheaper and has the same effect,
Culling of the weak*
Its a cool looking deck I really like culling of the night, itd be great with gravecrawler
Your Deck looks alot like the one i just made http://www.mtgvault.com/diablopally/decks/zombie-merchant/
I Think this deck needs 4x Silence 4x BaneSlayers 4x Journey to Nowhere 4x Oblivian Rings also if your worried about getting milled run a few little jaces =)