Porta San Donato was built in the 13th century on the road to Argenta and Ferrara and it was part of a bigger complex together with houses for guards. A drawbridge over the moat was built in 1354. The gate was closed and walled up for security reasons in 1428 but reopened again some decades later. Originally, there used to be another gate called San Giacomo built South of the gate San Donato, then walled up in the 14th century for security reasons. As the gate San Donato is located on the crossing of main streets (the ring boulevard and Via San Donato), it run the risk to be demolished several times, especially in 1952 and 1959 when finally only one metre of wall was destroyed after a heated debate to resolve traffic problems.
Source: "Restauro delle porte di Bologna 2007-2009"