I answered a similar question here , though more could be said.

Introduction

For this answer, I'm mainly going to be looking at your two latter questions with the example of Urban's Gun. Therefore, I'm going to look at why such a cannon was needed and if it was effective.

Discussion

The Basilic, or Urban's Gun, was the predecessor to the Dardanelles Gun and was used during the successful Ottoman siege and capture of Constantinople in 1453. Originally, Urban was attached to the Byzantine court in 1452, but, after they were unwilling to fund him, he left in favor of the nearby Ottoman sultan, Mehmed II. The sultan, who would soon be known as The Conqueror, was more than eager to fund Urban's massive cannon, which Urban claimed could smash through the walls of Babylon themselves.^(1)

So, why was Mehmed II drawn to this expensive proposition? Surely you don't just wake up one day and want to cobble together one of the world's largest cannons—unless you do! In which case, you do you. Besides the coolness factor, the Ottoman state had actually been bearing down on the floundering Byzantine Empire for some time and Mehmed II, was adamant about finally taking its fabled city, Constantinople. But to do that, he would need firepower and lots of it. For the Ottomans had laid siege to the city several times before, but had failed time and time again to even crack its glorious Theodosian Walls.^(2)

So, the defection of Urban seemed like fairy serendipitous timing! Especially considering that Urban had intimate knowledge of Constantinople's walls and all of its little, juicy weak points.^(3) And as Mehmed II really wanted to get the show on the road, Urban threw together his massive cannon in just months. But that was only the first part of its journey! While the cannon was constructed in Edrine, it still had to get to Constantinople. So with over 60 oxen and some 500 men, the cannon was finally dragged around 150mi (240km) to its final resting place outside the gates of Constantinople.

And I say "final resting place," because the gun was in quite bad shape after firing for some time. Most account place its breakdown within its first month of use. According to Nestor-Iskander, a contemporary account, the Ottomans even resorted to wrapping iron hoops around the cannon piece to hold it together as it fired.^(4) However, this fix doesn't seem to have worked, as the cannon immediately shattered upon its next firing.

However, while it was active, the cannon was rather effective. It managed to topple a tower and do some significant damage to the walls. Albeit, the cannon was rather unwieldy and the Ottomans seemed to lack skilled engineers to maneuver and properly use it. Besides just how difficult the cannon was to use, its accuracy was truly nothing to write home about and it often missed its target.

While we may imagine that this was fine as long as it could create a breach in the wall, in actuality, whatever breaches the gun made were quickly repaired by the defenders. With a total of three hours between firings, Byzantine and Latin defenders had sufficient time to border up any holes Urban's gun may have created. Therefore, we rarely saw what we may imagine: Ottoman soldiers pouring through some sort of collapsed section. Instead, it was a gate, which quickly became overwhelmed, that was the final breaking point before the city collapsed to the attackers.

But, there is one aspect of the cannon that some contemporary historians touch on: its boom. To say this cannon was incredibly loud would be the understatement of the year. The nearest defenders at Saint Romanus Gate would have heard the cannon fire as if it was thunder. And, once again, that is not an exaggeration!

In a digital recreation of the cannon, researchers found that the nearest defenders would have heard the cannon at about 120 decibels or about the sound of a thunderclap or being next to a chainsaw without ear protection.^(5) This is actually the point in which hearing something actually becomes painful. Even deeper in the city, such as at the Hagia Sofia, the cannon would still be clearly audible at 100dB.

Some contemporary historians, such as Pusculo, noted that this sound was unnerving to the city's inhabitants, who also saw its great plume of smoke tower over the city.^(6) Therefore, some modern historians have also interpreted Urban's Gun as a sort of psychological warfare.

Conclusion

Ultimately, the great cannon proved to be less successful than hoped and it surely didn't live up to the claim of being able to tear down the walls of Babylon. The cannon seemed to have scored some victories, but was inaccurate and difficult to manage and use. By the end of its first month of use, the cannon began to fall apart and there is no evidence of it surviving the siege. Overall, the cannon proved to be rather insignificant in the siege, although the mythos surrounding the gun has far exceeded its actual power.

Bibliography

^(1) Doukas 35.

^(2) In order: 1391, 1394–1402, 1411, and 1422, all of which were lifted.

^(3) Philippides' and Hanak's The Siege and the Fall of Constantinople in 1453, see page 395.

^(4) Philippides' and Hanak's The Tale of Constantinople (Of Its Origin and Capture), see pages 48-49.

^(5) Kakaliagos' and Ninis' Damage and failure of Orban’s gun during the bombardment of Constantinople walls in 1453, see page 488.

^(6) Philippides' and Hanak's The Siege and the Fall of Constantinople in 1453, see page 486.