Every child reaches a point in his life where he asks a grown up, "Why did the Empire spend all that time and money building the Death Star, only to leave that vulnerable exhaust port wide open?" That question marks the end of your naive childhood and the beginning of your life as a cynical adult.
But Star Wars was more true to life than you think. History is full of unstoppable super-weapons and fortresses that tended to have one, ridiculous flaw the enemy could take advantage of.
The Helepolis was the largest siege engine ever constructed in the ancient world. It was essentially a huge, rolling fortress--about 13 stories tall and weighing around 180 tons. It was used by Demetrius I of Macedon in his siege of Rhodes, about 300 years before the birth of Christ.
The name Helepolis literally means "destroyer of cities," which is badass enough, but it was also being driven by Demetrius, who had the nickname "Poliorcetes," which meant "The Besieger" in ancient Greek. Either Rhodes was in deep shit, or this was a case of gross false advertising.
Behind each of those windows up there was a catapult capable of launching an object heavy enough to turn a man into a puddle of spilled spaghetti. In total, it had two catapults on the first floor capable of launching 180-pound projectiles (basically picture them throwing refrigerators at you) and one catapult launching 60-pound ammo. On the next floor up were three more 60-pound catapults, then the next five floors featured 10 catapults that could rain down hundreds of 30-pound projectiles until nobody on the ground felt much like fighting any more. Finally there was a pair of dart throwers on the roof to kill any defender on the walls of a city.
Oh, and it was covered in iron plates, making the structure fireproof.
The Fatal Flaw:
Building-sized vehicles tend not to be all-terrain.
If you're ever facing something that 1) is on wheels; and 2) weighs 360,000 pounds, you can bet on one thing: It's not going to be worth a shit in the mud.
The Rhodians made an educated guess about where the attack would come from, and before the rolling tower of badassery could get there, they channeled water and sewage from the city and turned the whole area into a bog of mud and human poop. When the Helepolis moved in for the kill, it got bogged down in the mire and a tower full of soldiers realized there probably weren't enough horses in the world to drag the their rolling fortress to dry land. They abandoned their superweapon.
History does offer an alternate, equally sad, version of the story where defenders were able to simply run up to the thing and pry off the iron plates, aka Luke Skywalker bringing down a walker by knocking a hole in it with his lightsaber. Everyone agrees the outcome was ridiculous either way.
Without the Helepolis, the siege of Rhodes failed. The so-called "Destroyer of Cities" was abandoned, having never destroyed a city. The Rhodians tore it down and used parts of it to build the Colossus at Rhodes, which was not in fact a big-ass superweapon, but just a huge statue of the Greek god Helios positioned so that every passing ship would be forced to look up at his junk.