In 1499 Krakow was a wealthy city under constant threat of attack, especially from the rampaging Ottomans. So they made themselves into a fortress. The Great Barbican is both the principal entry point to the city and a massive seven turreted point of defense. These days it looks like a fairytale city gate, back then it was either a massive relief to reach it with your wagons intact, or a deterrent to your planned attack on the city.
The actual gate to the city was St Florian's gate, linked to the Barbican by a covered passageway. But the Barbican and the series of moats and walls which lead away from it, ringing the city, were the first point of entry to Krakow in the Middle Ages. Today, you still enter the Old Town of the city through the impressive Barbican.