Here is the list of musical themes representing towns. Music for nine traditional towns is composed by Paul Romero, while the theme of Cove is written by Horn of the Abyss founder Docent Picolan.
Castle
The theme of Castle is a solemn and noble melody in A major. Woodwind trills resemble horses' neighing, which is a reference to Cavaliers as well as to Knight heroes, while bell toll is an allusion to Monks and Cleric heroes. Castle theme is related to the main menu theme, which is written in the same key.
Rampart
Rampart has possibly the mildest town theme in the whole game. It is a Celtic-like melody accompanied by a harp, written in G minor. Rampart theme has a brief intro and ending, both featuring melodies by wind instruments.
Tower
The theme of Tower (D minor) is the longest and most developed in the game. It's a cycle of variations on a waltzing melody which gets more and more courageous and militant. Fast virtuoso passages depict flying snowflakes, since Tower's native terrain is Snow, while using of choir and percussion creates an enchanted atmosphere, as Tower is one of the most magical Heroes 3 towns.
Inferno
Inferno's music illustrates the element of fire. The main melody in A minor consists of fearsome exclamations, with exceptionally scary sound of brass instruments, while the accompaniment depicts boiling lava. The whole theme of Inferno is dramatic and restless, like a theme of hell should be.
Necropolis
The theme of Necropolis is a realm of horror and dread. It starts with funeral bell tolls and a moaning choir melody (C minor), which seems to be a choir of dead souls. Then comes a famous Gregorian chant "Dies irae" ("Day of Wrath"), which is a symbol of death in many classical music works. It is followed by warlike music which gets more and more sarcastic, reminding a tango. In the end, "Dies irae" is performed by harpsichord in B-flat minor, driving deep sorrow and depression.
Dungeon
Soundtrack of Dungeon is dark and gloomy, but not as dreadful as in Necropolis, providing some sophisticated oriental motifs. A melody in the rhythm of a slow waltz (A minor) develops throughout the piece, getting more and more frightening. It is interesting that Tower, which is Dungeon's fierce rival due to Titans-Black Dragons stand-off, also has a waltz-like theme.
Stronghold
Stronghold theme is written in F minor, and is one of the most dramatic and militant in Heroes 3. It is in fact a scene of a tough battle, with many specific effects depicting Stronghold's creatures, for example, Thunderbirds' shrieking, Goblins' crude speech (notice sound effects when visiting their external dwellings), and unstoppable gallop of Wolf Raiders. Stronghold is a symbol of brute force, same for its music. Originally this theme was created for Castle.
Fortress
The theme of Fortress is notably the only town theme written in neither major nor minor key, being based on augmented triad, although G-sharp minor starts to prevail as the piece gets developed. A bizarre and sophisticated melody is followed by battle music somewhat resembling the theme of Stronghold. This combination of heroic and magical music goes well with Fortress which doesn't have top level Mage Guild, but features many units with special magical abilities.
Conflux
The soundtrack for Conflux is about peace and serenity. It is the shortest town theme, and one of only two written in a major key, the other being Castle (Conflux's key is C major). There is no big contrast within the theme of Conflux, which depicts the world of pure elements and emotionless Elementals. Fanfares aren't that militant since they are softened with harp.
Cove
Music of Cove is written by Docent Picolan instead of Paul Romero. It may resemble Conflux theme, as they both aren't too dramatic, and Cove's main melody is even quite lyrical. The key is C minor. The theme of Cove is accompanied by rumble of waves, while previous nine town themes don't have any sounds of nature itself (in the official editions, only terrain themes have it, for example, the one of Water also features splashing sea waves).