Festung Hohensalzburg sits on the Festungsberg, a limestone outcrop rising 506 metres above sea level — roughly 150 metres above the old town of Salzburg below. It was begun in 1077 by Archbishop Gebhard von Helfenstein, a Gregorian-reformer prelate who needed a defensible residence during the Investiture Controversy — the great church-versus-state war between the papacy and the Holy Roman Emperor Henry IV. Successive prince-archbishops expanded it for the next five centuries; the version visitors see today was substantially completed under Leonhard von Keutschach around 1500.
It is the largest fully-preserved medieval castle in Central Europe and was never taken by force in over 900 years of standing — though it has been surrendered, notably to Napoleonic forces under General Moreau in 1800, without battle. Successive prince-archbishops invested heavily in walls and outer defences; the only time it actually came under siege, during the German Peasants' War of 1525, the attackers failed to take it. Inside the walls today are the late-Gothic Princes' Chambers (Fürstenzimmer) with their original wooden ceilings and tile stoves, the Magic Theatre and its 16th-century mechanical organ known as the Salzburg Bull, the Marionette Museum, and a working funicular — the Festungsbahn, in operation since 1892 — that connects the fortress to the old town in about a minute.
The fortress is open every day of the year. Tickets are valid six months from the date of purchase, so there is no slot pressure on the booking itself; the pressure is on the funicular, which queues 30 minutes or more between 10am and noon during the July–August peak. Skip-the-line tickets bypass that queue and put you on the next available car.