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How were the Latin letters c, qu, and gu pronounced in Classical Latin?

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Italic languages Map showing the approximate extent of the Italic and neighboring languages about 250 bce.

In Classical Latin, the letters c, qu, and gu represented specific velar and labiovelar sounds, distinct from their later developments in Romance languages.12

c: During the Classical period, c consistently represented the voiceless velar stop /k/, regardless of the following vowel. The "soft" pronunciation of c (as /s/ or /ch/) that appears in modern Romance languages resulted from a later process of palatalization that had not yet occurred in Classical Latin.1
qu: This represented a single labialized velar consonant, /kʷ/. It was not a cluster of two separate sounds, as it did not contribute to the weight of a syllable in the way a consonant cluster would.2
gu: Similarly, gu represented a single labialized velar consonant, /gʷ/. This sound occurred only after /n/.2

These sounds were part of a broader consonant system that included both simple velar stops (/k/, /g/) and labiovelar stops (/kʷ/, /gʷ/).2