The third was undertaken between the end of the fifteenth and the beginning of the sixteenth centuries, based on the existing model. However, considerable advances in military technology lead to the decision to effect interventions of improvement and fortification, given that the existing structure was designed more for defending the town from long shots with a falling trajectory than from the devastating force of direct shots. Not to be overlooked was also the fact that the dominion of Florence at that time went as far as Altopascio, a mere 15 kilometres away from Lucca. Thus in 1544 a construction site was set up and work got under way directed by experts from various parts of the country (in particular Urbino) with the help of Flemish technicians.
The number of diggers, carters, carpenters, furnacemen, blacksmiths and builders needed daily was impressive, up to 2,000, to the extent that for the more simple jobs, a sort of obligatory enrolment was set up for the people of the surrounding countryside, the so-called 'fatigue-duty' organised on a daily or weekly basis. Work came to an end after an enormous expenditure of time and money, over a hundred years later, in 1650. The circuit is composed of twelve ‘cortine’ with ramparts which join up between them eleven bastions, nine of which in the form of a projecting spur fitted with trunnions (in the style of a type that was in use in the mid-sixteenth century); one, that of S. Maria with square flanks and one, S. Frediano, resembling more a platform, the whole making up four kilometres and two hundred metres of fortification with a scarp wall, 30 m wide at the base, on which were placed 124 pieces of artillery.
Large tiers of earth lead down to the town, on which trees were planted in order to consolidate the embankments and also have wood at the disposal of the town in the event of a long siege. The defensive system on the exterior was reinforced by digging a 35 m wide ditch of twelve demi-lunes of earth with foundations in masonry (two of which, the only ones left, are present in the part between the platform of S. Frediano and the Bastion of S. Donato), and a long continuous embankment and lastly by cutting down all the trees within a range of half a mile (area known as the 'felling' or 'clearing' line), so as to not leave timber at the disposal of the enemy for the artillery. The constructions along the walls, known as 'casermette', were built to accommodate the guards.
The only alarm that the walls have had to face was that of the water from the river Serchio which in 1812 nearly flooded the town. All the gates into the city were closed and reinforced; fortunately the town remained unharmed. By this time, the walls had taken on a less military aspect as the Austrians during one of the rotations of troops in exchange with the French army after 1799 had taken away the cannons. Following the Congress of Vienna, the new Duchy of Lucca was entrusted to the Borbone family of Parma, specifically the Duchess Maria Luisa who appointed the architect Lorenzo Nottolini to covert part of the walls back into a natural green area again. This converting back of the old defence system to a more civil use was further accentuated when in 1840 the Caffé delle Mura was built on the Bastion of S. Maria, then demolished and rebuilt further back in 1885 in order to create a square with the statue of Vittorio Emanuele II, directly connecting with the town centre. In those years the idea of transforming the area of the walls into a public park began to take form.
The entrance-gates originally were only three: S. Pietro, S. Donato and S. Maria. Porta Elisa, of neoclassical style, was inaugurated in 1811, Porta S. Anna in 1910 and Porta S. Iacopo (or IV November) in 1930. In 1866 at risk of it being sold to private persons, the Comune decided to buy the area of the town walls from the 'Regio Governo' to whom it belonged as State property in the form of a defensive structure, which fortunately helped guarantee their survival as a historical monument.