and...

I'm a little unsure whether this is fact or not but, I was told as a kid that the kill floor was on the top of the packing houses.  This was to allow the animals to 'power' themselves (walking) up the ramps to the highest level, and once killed, gravity would assist their return trip to lower levels while hanging from a rail.

Building the pens upward might been a way to have more livestock readily accessible to the entrance point of the kill floor?

I remember going along for the ride when delivering loads of fat hogs to the stockyards at St. Joe as a kid, and driving under the big ramp that went across the street from the holding pens at ground level to the top of the kill plant. 

Never got to (or wanted to) tour a kill plant back in those days, so not sure about any of the validity of what I was told.  But it does sort of make sense.

Seems like there may have been a 'documentary' of some sort years back that addressed the use of gravity, to assist in the movement of carcasses 'back in the day', but I can't remember where I'm getting that particular memory from.