twoerofpisa.org says the tower was intended to be plumb, but went off plumb during construction. That may be true, my observation though is that the bell tower, the top level, is obviously canted compared with the rest of the structure, and at least THAT was deliberate!
Same website describes the other structures. It is a Christian site.
Finally, "ancient" is overstating things - the cathedral is old, dating to a similar time as the tower. From memory, there are older artifacts in that area, certainly the cemetery adjoining.
Not shown in the picture, and maybe different today, the "other side" of the road is a tacky tourist trap. Souvenir shops, photos, post cards, tee shirts.
I went there when I was a kid. I vaguely remember that area. Thank you for info.