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The walls of old Jericho have been dated to around 8000bc. This is thousands of years before the nascent civilisations of Babylon and Egypt built similar fortifications and thousands of years before organised warfare made such walls useful for defence.

Walls thirteen feet thick with a watchtower and a moat are simply unnecessary to keep out the random raiding parties which characterised warfare in this era. Which rather raises the question of what they were for?

The current Wikipedia page ponders this and posits answers around the old chestnuts of a ritual or ceremonial function

Ran Barkai argued that the structure was used to create awe and inspiration to convince people into a harder way of life with the development of agriculture and social hierarchies.

But unless I've missed something, this seems extremely unlikely. The wall included a ditch or moat excavated, at what must have been a vast cost in labour, out of solid bedrock. Given this addition, it is hard to believe that the purpose of the wall was not primarily defensive.

But what on earth could the ancient inhabitants of the city have needed such impenetrable fortifications against in the Neolithic era?

The short answer (unfortunately but unsurprisingly) is that we can't be sure. However, the currently most accepted theory would appear to be that the walls were for flood control, but there are dissenting views. The tower, on the other hand, has been associated with the summer solstice, among other things.

THE WALLS

The most likely explanation would seem to be the walls were to protect the city against floods.

The first known fortifications built around a settlement appear at
Jericho during the Pre-Pottery Neolithic A, around 8000 B.C.(9) The
nature of the walls is debated, however,and it has been convincingly
argued that they were intended to protect the settlement against
seasonal flooding.(10) Evidence indicates that the military threshold
for a protective wall was not crossed in the 8th millennium.(11)

The sources given above are:

• (9) O. Bar-Yosef, The Walls of Jericho. An Alternative Interpretation, Current Anthropology 27, 1986, 157–162


• (10) K. M. Kenyon, Excavations at Jericho, The Journal of the Royal Anthropological Institute of Great Britain and Ireland 84, 1954, 103–110


• (11) W. J. Hamblin, Warfare in the Ancient Near East to 1600 BC. Holy Warriors at the Dawn of History (London 2006)

Bar-Yosef, who cites over 30 sources, concludes that

Given all the available data, it seems that a plausible alternative
interpretation for the Neolithic walls of Jericho is that they were
built in stages as a defense system against floods and mudflows. The
PPNA inhabitants of Jericho chose to live near a copious spring on a
sloping plain which was subject to mudflows and sheetwash. Their
response was to build a wall and then, when necessary, dig a ditch.
The necessity for better protection on the western side would explain
the varying thickness of the wall, which during Stages IV-VI was ca.
3.5 m in the west but remained only 1.4-1.6 m in the north and the south.

The varying thickness of the walls could suggest their purpose was anti-flood rather than defensive. Also, during the later PPNB period the wall was not maintained to the same potential defensive effectiveness. Changes in the likelihood of floods seems a reasonable explanation for this.

There are, though, dissenting views which argue that the wall was to defend against a human threat. Hamblin, while accepting that there was "a lack of serious and sustained military threat in the early Neolithic" says:

The appearance of such massive fortifications a thousand years before
fortification in other regions has led some to question their purpose,
claiming the walls were designed to protect the community from flash
floods out of the wadis to the west. However, it seems dubious that
protection from flash floods would requiresuch a massive
four-meter-high wall – indeed the ditch alone should have
provedsufficient for flood control. The stronger interpretation is
that the wall and tower had a military purpose....It is likely that the Neolithic fortress of Jericho was built in response to a very specific, local, but ongoing threat

As with the flood theory, there is much speculation here. Hamblin's inclusion of the tower doesn't make much sense given it's positioning in relation to the wall (it's defensive use is limited).

THE TOWER

On the tower, Bar-Yosef notes that

The later history of Near Eastern fortifications seems to rule out its
use as part of a fortress.

but he comes to no certain conclusion as to it's purpose:

The archaeological remains indicate that the tower was a special
structure and perhaps held a special place within the settlement.

He goes on to say that:

The presence of the storage facilities attached to it in its early
days may hint that it was publicly owned or at the service of the
community. It is quite possible that it was also a place or a center
for ritual activities.

So, yes, that 'old chestnut' ritual crops up again but, in the absence of any firm evidence, this is hardly surprising. Archaeologists Ran Barkai and Roy Liran, after looking at the surrounding environment and analyzing the architectural design have concluded that

the tower is in fact inherently aligned to celestial and geographical
elements, and that the ancient Neolithic builders used it as a link
between them, their town, and the universe.

Archaeologist K. Kris Hurst, citing Barkai and Liran, writes:

The stairs at the top of the tower open up facing to the east, and on
what would have been midsummer solstice 10,000 years ago, the viewer
could watch the sun set above Mt. Quruntul in the Judean mountains.
The peak of Mount Quruntul rose 350 m (1150 ft) higher than Jericho,
and it is conical in shape. Barkai and Liran (2008) have argued that
the conical shape of the tower was built to mimic that of Quruntul.

This Jerusalem Post article (2011), citing an interview with Barkai, adds:

Barkai said architecture designed to awe and inspire, and without any
obviously functional purpose, isn’t unique to the megalithic period.
Even today, governments erect monuments like the Arc de Triomphe to
influence public opinion and enhance their standing.

Acknowledgement: Mazura for the lead provided by his comment on Barkai and Liran.

There may be a slight possibility that one purpose of the walls of Jericho was protection against elephants.

Asian elephants are normally shy and tend to hide from humans. However, there are examples of elephants who react in a very human manner to various human attacks and aggression.

Specifically there have been elephants so angered by humans that they have hunted down and killed humans.

The now extinct Syrian subspecies of Asian elephant was the largest subspecies of Asian elephant.

So possibly one reason for building the first wall of Jericho was defense against one or a group of vengeance-crazed elephants seeking to kill every human they could catch.

The tower might have been a watch tower to look for approaching elephants and sound the alarm to warn people to head for the safety of the walls and perhaps to send out warriors to chase away the elephants.

Another possible reason to build the wall might have been defense against migrating herds of large animals that perhaps had migration route through the area. A migrating heard of aurochs that stampeded would be very dangerous, and the walls of Jericho would have been a sufficient defense against an aurochs stampede.