庭には二羽鶏がある

[Belton]

I've been having a conversation about this tongue twister.
にわの庭には二羽の鶏は俄に鰐を食べた。
にわのにわにはにわのにわとりはにわかにわにをたべた。
In (mr)Niwa's garden two chickens suddenly ate a crocodile.

Now, I found this on Japanese wikipedia.
But it seems the first phrase にわの庭には doesn't make grammatical sense.
Shouldn't it be にわの庭で？
If it does make sense, why the second は and not が？

Has someone just expanded the original 庭には二羽鶏がある without thinking?

http://ja.wikipedia.org/wiki/早口言葉
(the Japanese characters don't parse into the link, you'll need to do a cut and paste.
hexcodes if I could do it would make for a long link I think)