To answer this question, I would look to some of the greatest fortifications of the medieval period - one of them just so happens to be an area of expertise: the Theodosian Walls of Constantinople.

Constructed in the 5th Century on the orders of Emperor Theodosius II and built by Praetorian Prefect Anthemius, the Theodosian Walls were the grand bulwark that defended the Imperial Capital from hostile invaders for over a thousand years. On the land side, the fortifications consisted of a ditch, a wide moat, and three layers of walls.

The ditch was approximately 6 feet deep and was filled with a variety of devious traps, such as caltrops, pitch/Greek Fire, and spikes. In front of the ditch was an abrupt drop off into the moat - itself 61 feet across and approximately 20-25 feet deep, presenting a significant obstacle for attackers just on its own.

At the far side of the moat was the first wall (known as the μικρόν τείχος, meaning "little wall") rising directly in front of the water, and is approximately 6 feet high, not including crenellations. Defending archers and ballistae could easily set up behind these low defenses and shoot essentially at ground level across the moat to kill attackers, but the primary purpose of this small outer wall was to add an additional obstacle to surmount even after crossing the moat. Inside of this little wall was the parateichion, which means "wall divider", and was a barren killing zone filled with additional traps - again, caltrops, spikes, Greek Fire, etc.

Next was the Outer Wall (ἔξω τείχος), approximately 27 feet above the parateichion in front of it, about 7 feet thick at its base and heavily strengthened by an arched construction (which also provided small camping spots on the inside of the walls for defenders if necessary). Small outcropping towers approximately 35 feet high allowed for various ballistae to be mounted on top and would have easily allowed defending archers a clear shot at enemies attempting to take cover next to the base of the walls. The bottom of the walls were also rounded and slick, further preventing enemies from taking cover and making the mounting of ladders more difficult.

Inside of the outer wall is the peribolos ("enclosure"), presenting an even more daunting killing zone, again likely laced with traps when deemed necessary. Behind this was the final and most impressive of the Theodosian defenses - the Inner Walls (μέγα τείχος). Measuring over 40 feet high and over 15 feet thick at their base, they were also dotted with 96 evenly-spaced 60-foot-high towers of varying shapes (octagonal, hexagonal, and square) which provided additional mounting platforms for various ballistae, and even perhaps fire projectors. These too projected in front of the wall line, allowing archers and ballistae clear shots at attackers. Secret tunnels were also built within the walls allowing troops to easily move from one section of the wall to another.

So while you might not see a single wall a hundred feet thick, you would definitely see in the best cases, extensive multi-tiered fortifications that could cover a "defensive zone" hundreds of feet thick.

With unlimited time and resources walls could become infinitely thick. The thing is that width is not a problem in construction of walls its the height. Height with masonry has compression issues that cannot easily solved. That's why the flying buttress was invented. I am not a structural engineer so i cant tell you the exact physics of masonry and the height a stone wall will top out at before it can no longer support itself. If your curious about large castles of the 11th century i would recommend reading up on Caernarfon Castle in Wales. Its one of the bigger castles of that era. I would also recommend the Theodosian walls of Constantinople. They were built 600 years before your time period but are quite a feat in themselves.

The Chateau de Ham in France built in the late 15th century had walls 35 feet thick the thickest in Europe. The average castle would vary between about 12 feet and 20 feet. But Europe's castles are dwarfed by massive walls built in other places. In Japan Osaka Castle built in the early 17th century has walls 57 feet thick.

But both of these walls may be dwarfed by the famed "Walls of Babylon" These were written of by the Greek historian Herodotus. Now he may have been exaggerating but if you believe him the walls were about 350 feet high 85 feet thick and 50 miles long. There were three walls. The inner one protected the city itself. The outer one enclosed enough arable land that the city could hold out a siege indefinitely. The walls were large enough that chariot races could be held on top of them.

As I said Herodotus was probably exaggerating a bit but not about the chariot races that part is true. The walls were probably closer to about 100 feet high including towers and about 40-50 feet thick.

Walls hundreds of feet thick are probably more than you would ever want for a castle. I mean really if you build your walls too thick you stop building a castle and instead have constructed a mound or a hill. Something has to still fit inside of your castle when you are done building walls around it.

For a city I think the Walls of Babylon are a pretty good example of what you can do with unlimited resources. Hundreds of feet thick is a bit of a stretch but too large for a historian to measure is well within reason.

Source

The Readers Companion to Military History edited by Robert Crowley and Geoffrey Parker