Sparta had
not formed into a city yet either during this period. Sparta consisted of 4 very close villages (Limnai, Kynoussoura, Messoa, Pitane) and a 5th (Amyklai) about 6 km to the south. The lack of Sparta's walls has more to do with it being quite backwards and traditionalist compared to much of Greece. It was also in a fortunate position that it did not have to build them. It was a
polis, but not a real
city yet. Some defenses were built surrounding most of the polis in the early 3rd century, with a full wall being built only by 209 BCE. However these did not cover Amyklai which nontheless remained part of the polis, but not the city.
Now beyond the source based fanboy reasoning for Sparta not having a wall,
why did they not build walls? The sheer size of surrounding the entire area would have been a very
costly and time consuming effort, especially when Sparta could rely on the
perioikoi, that is to say the other Lakedaimonian poleis which were subservient to Sparta politically. It meant it could afford to remain old fashioned and not have walls because it had the perioikoi as its forward defense. Once it started to lose its subordinate poleis and helots, Sparta
had to defend itself at the expense of having Amyklai excluded from from these defenses. Sparta had been so reduced in size (to that of an average polis) it could not afford this tradition anymore.
I think a
better representation could be to
carve out the Mountain province of Pellana or Sellasia to the north of Sparta, with a fort there, and to have a fort to the south of it in 430 Gytheion. Perhaps something as
@Ketchup & friends proposed above could still be the case, with a big spike in unrest for its Citizens and Nobles if the option to have a fort is taken. This is to represent the likely unrest it would have caused if Amyklai was suddenly cut off from the rest of the polis by a wall which would likely anger the backwards and traditionalist Spartans.
I disagree with giving them yet another morale and discipline boost however. They already have one from their Heritage, and compared to how they historically fared are already really powerful through their missions. The trade off should be having a free wall and a spike in Citizen and Noble unrest vs not having one and keeping the peace. Perhaps something similar can happen if Sparta builds a fort there when it is doing well. After all why change the tradition if Sparta is not weak?
EDIT: Not saying it should not be a city by the way, the way to represent the 4 villages, is by a wall-less city.
Also: If another province is not carved out, then Sparta having a fort is fine, the fort can represent that part of its perioikoi.