Why didn't ancient Rome have city walls at the empire's peak?

Answer: The legions WERE the wall — defense was offensive depth

The legions WERE the wall — defense was offensive depth ✓ — Correct! For 600+ years, Rome's defense doctrine was strategic depth: enemies were stopped at the frontier (Hadrian's Wall, Limes Germanicus) by professional legions, hundreds of km from the city. A wall around Rome itself would have meant the empire had failed. The Aurelian Walls were only built in 271 AD when crisis-era Rome admitted it could no longer guarantee the frontier — a stunning reversal after 700 years of wall-less confidence.

Roman engineers couldn't build walls big enough — Wrong. Roman engineers built the Colosseum, the Pantheon, and 80,000 km of paved roads. Walls would have been trivial. They CHOSE not to build them — confidence in the legions, not technical limits.

Rome had no enemies between 100 BC and 300 AD — Wrong. Rome fought constant wars throughout this period — Parthia, Germania, Dacia, Britannia. The point is those wars happened FAR from Rome. Defense in depth meant fighting was always somewhere else.