It looked profoundly unimpressive. Thucydides famously points this out in his archaiologia, the introduction to his work where he outlines the early history of Greece (1.10.2):

I suppose if Lakedaimon were to become desolate, and only the temples and the foundations of the public buildings were left, that as time went on there would be a strong disposition with posterity to refuse to accept her fame as a true exponent of her power. And yet they occupy two-fifths of Peloponnese and lead the whole, not to speak of their numerous allies outside it. Still, as the city is neither built in a compact form nor adorned with magnificent temples and public edifices, but composed of villages after the old fashion of Greece, there would be an impression of inadequacy.

The city of Sparta was composed of five rural villages on the banks of the Eurotas, four clustered together and a fifth a few kilometers to the south. The inclusion of this fifth village (Amyklai) in the community is the probable reason why the "city" wasn't encircled by a defensive wall; that wall would have required an enormous investment in labour and stone. Archaeological evidence suggests that many Greek city-states congealed in this way out of smaller rural communities during the Archaic period, eventually fortifying themselves in the 7th or 6th century BC, but Sparta didn't take this step until the Hellenistic period.

The result of this lack of true urbanisation and fortification was that Classical Sparta remained little more than a few scattered clusters of houses around an agora, an open public space used for meetings and trade. Sparta's lack of public funds meant that there had been few resources available for the construction of major temples of the sort other Greek states had started building in the 6th century BC. Its emphasis on the personal rather than financial contribution of citizens to the state meant that there was little in the way of public buildings (council halls, stoas, fountain houses) or private monuments either. Athens in the 5th century BC spent lavishly on the fortification and beautification of its public spaces, completing famed projects like the Long Walls, the Parthenon, the Painted Stoa, and the Erechtheion. Sparta, on the other hand, increasingly embraced a public image of austerity and moderation, in which such extravagance had no place. They also seem to have started to present their lack of walls as a sign of their citizens' courage, instead of the backwardness noted by Thucydides. A temple of Menelaos, an early incarnation of the temple of Artemis Orthia and a grave monument that formed the centre of the heroic cult of Leonidas were some of the only monumental buildings in the urban centre of Sparta.

The rest of the city would have consisted mainly of the sort of houses seen throughout the Ancient Greek world. These were largely single-storey or two-storey complexes built out of mud brick around a walled courtyard, with small windows only on the top floor. Blank walls with doorways would flank irregular, winding streets in cities built in the old, Archaic style (planned new settlements would generally be laid out along a Hippodamian grid pattern). There would not be much evidence of the famous wealth of Sparta in the modest, uniform appearance of the houses and the limited use of monumental materials like ashlar masonry and marble.