why is there a bell in blighttown

Never thought of this but why put a bell in blight town when it a utter hell hole ??
 the bells are there to test an undeads ability to see if he ready for sens but how they know there be a guardian to test the undead there to begin with.

the other bell is guarded by gargoyles creature who are part of gywns army or magical constructs. they always going to be their to guard it until they died how they(the bell builders) know that someone going to guard the bell in blight town unless they somehow made a deal with queelag which begs this question which came 1st the creepy naked spider chick or the bell

the only answer I can see is frampt/someone told queelag if u stay and guard this place many undead with lots of humanity's come for you to kill 2 take humanity's and heal her sister but then again how frampt/someone know about her sister

They were built to be a key to Sen's Fortress, I think.

The builders may have known about the difficulty of getting through Blight Town already, as well as how the inhabitants would have reacted if they were to see an undead encroaching in on their territory.

Now as far as Quelagg goes, she may have moved in after in order to collect humanity from the wandering undead and other undead thinking they are the "chosen undead" so she could heal her sister. She probably wasn't originally there when they were constructing the bell tower.

Blighttown didn't start as Blighttown....it only became infected when a strong pyromancy was released. I imagine that the bell didn't start there, nor was Sen's Fortress always there. They were both added when the Age of Fire started to fade and the "prophecy" was introduced. As mentioned, they are a test.

I also figure that Quelaag wasn't placed there by the serpents or the gods. I imagine she simply took up residence when her sister became poisoned and she realized the potential of the humanity she stood to gain as adventurers happened by.