Oh, hey, this is super late but I just saw this thread and I have an answer, if you're still interested and willing to take Suetonius' word for it!

In particular, for the adornment and convenience of the city, also for the protection and extension of the Empire, he formed more projects and more extensive ones every day: first of all, to rear a temple of Mars, greater than any in existence, filling up and levelling the pool in which he had exhibited the sea-fight, and to build a theatre of vast size, sloping down from the Tarpeian rock; to reduce the civil code to fixed limits, and of the vast and prolix mass of statutes to include only the best and most essential in a limited number of volumes; to open to the public the greatest possible libraries of Greek and Latin books, assigning to Marcus Varro the charge of procuring and classifying them; to drain the Pomptine marshes; to let out the water from Lake Fucinus; to make a highway from the Adriatic across the summit of the Apennines as far as the Tiber; to cut a canal through the Isthmus; to check the Dacians, who had poured into Pontus and Thrace; then to make war on the Parthians by way of Lesser Armenia, but not to risk a battle with them until he had first tested their mettle.

Suetonius' Divus Iulius, translation by the Loeb Classical Library via LacusCurtius.