So, I've played very few games of D&D, and most of those were through Skype. Well, since the last time I read a rulebook was way back in 2002, I was allowed to make a Spellsniper class (I'm sure I'm using this incorrectly, but basically I was an archer who could cast low level healing spells, and some destruction spells when using my bow) whose home town was burned to the ground by the campaign's Big Bad. When we got to the second to last dungeon, we encountered the BBEG, and he offered us a final chance to not have to fight him. I got into an argument with him, and through sheer luck, charisma, and some high rolls, I managed to buy his entire Evil Fortress of Doom and Stuff for only two metric tons of gold (Our GM might've been a little on the generous side. I mean, what kind of shopkeep buys a rusted pickaxe for 50 gold?!). After that, we set up shop in the Evil Fortress, and started converting/exorcising/disbanding his armies until they were all back to their regular selves. After that, I made a treaty with the King and got all of us Knighted, as well as a princess each. The BBEG didn't realize that half the gold had been transmuted from coal until after he moved into the volcano lair the GM had had to set up. It was a simple thing to do to mobilize our 50k strong army and defeat him after that, and we didn't have to destroy an honestly well detailed fortress.

Ah...

My players managed to take over the world at one point...

Granted, saiid world consisted of only a couple of nations on an isnland continent maybe the size of great britain, but still. Not what i had planned for the campaign.

They were sent on a suicide mission to kill the commander of the enemy nation, ended up leading the resistance and taking over the show in it's entirety. Then they proceeded to take over their home country and that was really that.

Game setting, system, GM, and party composition meant we didn't do it that often, but for our GURPS games at least, it was usually just a matter of someone "wanting" to play the independently wealthy rich eccentric, at least with modern conveniences. In a setting where you can't instantly communicate with your employees it became a lot riskier. Throw in how hard it was to run in a lot of historical or fantasy/medieval settings where social norms were often different and sometimes flat out against player/character convictions, and it often wasn't worth the hassle.

Oh, and there were risks from things like trying to use modern business practices substituting magic for technology; botch e-mailing an important file and you might delete it instead, send it to the wrong target, maybe even activate some malware. Botch a spell sending information and not only do you risk the above, but malware is changed to "malevolent entity", including but not limited to rival spell casters and demons.

Town? City?
In one campaign our group tried to take over the world.
The group subjugated a few primitive orc tribes, molded them into an organized army and started invading the smaller countries in the vicinity. It quickly ballooned from that...
...until my rogue - who was probably the most "good" aligned in the party - formed a resistance movement among the soldiers that had been pressed into service from subjugated countries - there were a lot of people who were not too happy with being under the rule of people who had gotten into power with the help of orcs.
A bit of fast-talking the GM might have been involved as well...

Tale 1) Through trickery, massive luck, and some skill, a trio of players formed their own massive galactic 'commonwealth', ruled by one and the other two running the space and ground militaries respectively. Mainly because the ruler was the 47th son of royalty and staring from his homeworld, eliminated all competitors, and through alliances and capitalism attained several star systems more to form his own little Slice of the galaxy.

Tale 2) Less grand scope, just an awesome ship, and some major deceit. Not 1, but 2 players spent the entire game independently plotting to destroy the rest of the party (including each other), they plotted and schemed for an entire semester before almost simultaneously springing their plots, destroying the ship they'd all been on the entire campaign, and then at the last moment, as the ship was going down in flames... somewhat literally as parts of it were effectively napalmed... they joined forces so they both would survive in the smaller, faster, more heavily armed ship in the cargo bay, instead of fighting to see who would survive (and most likely guaranteeing both of their deaths instead). BTW, both planned to betray the other at the next starport, space station, asteroid base, or orbital platform, if the campaign had been renewed the next semester... except the other four players just wanted to make characters to get revenge, so we decided to end it there... sort of a shame since both of them had some really evil ideas on how to take the other out.

Recently I had a Tech Priest and a Psyker take over an enemy ship during a boarding action. We blew out their boarding tube (taking out many of their marine-types) and crossed through the void to board them in retaliation. We then rushed our way to their bridge and I killed the captain while the Psyker dealt with the rest of the bridge crew. I then hacked the controls and ended the fight with the bridge overrides, resulting in a prize crew being sent over.

More pertinent to conquering a city, though was our Rogue Trader campaign (our first foray into FFG's 40k RPGs). Over the course of the campaign we went from an old but failing Rogue Trader house to the dominant force in the Koronus Expanse (complete with destroying/absorbing our rival house entirely). We parleyed our wealth and connections into several strategic marriages (our captain married the queen of the capital planet for the neighboring sector, and my character married the daughter of the sector governor) and some key political alliances (we got some very potent contracts with the Navigators and made very nice with a powerful Forge World). My character also got himself declared a Living Saint, which gave him some extra influence. Finally, we had a psyker with lots of telepathic powers, including plenty of mind control, which he used to get us even more political control.

In the end we wound up taking over the Kalixus sector, founding the Koronus sector, and pretty much influencing every subsequent 40k RPG campaign we've played (our old PCs being at times allies and at times enemies).

40k lends itself to ridiculous outcomes, and several of our players are rather... extreme in their munchkin tendencies. Thankfully our GM is both understanding and capable of adapting his plans to whatever crazy stuff we get up to, and it's really payed off.

Our Chaotic Neutral arcane caster BURNED DOWN an entire town by turning himself into a large fire elemental and then rolled max summoning for fire elementals (1d4+1). All to get rid of this corrupt church shilling the townspeople. Does that count?

(Also: He flew above an opponent in the middle of the blaze [freakin' Aasimar] and turned himself into an Earth Elemental to end the battle. Complete with a loud shout of "CANNONBALL!!!" I don't think ANYONE saw that one coming.)

In an earlier campaign than the one with the evil cleric, One of my friend's low-level paladins became the prophesied True King for a city-state that hadn't had a legitimate ruler in a long time. Instead, it was governed by a warlord with his higher level henchmen as a junta.

When the paladin reached 9th level, I had the word spread in the city that the rightful king had returned. The citizens revolted and chased the junta out of the city. The paladin then had to defend himself against the old head of the thieves guild, an evil patriarch, and an evil sorcerer, before he could face the warlord. He killed them all in single combat in their jungle lair. Good thing he had more hit points than all but the warlord, was resistant to magic because he had a holy sword, and had a better AC.

Once that was done, he was able to rule his city-state in relative peace.

Please, I once played a wizard who plotted to have himself declared king of an entire nation. The throne was conveniently empty after we revealed the "queen" as an evil shapeshifter.
The man who ruled in her stead was gullible and I had an incredibly high bluff check. Soon, he was unconscious, naked, and tied up in his own closet, I was disguised as him, and I was all ready to announce the next successor to the throne. Hint: It was me.
It would have been perfect, except the DM had a nervous breakdown and never spoke to me again.

We were attempting to take over a city to build our fortress of evil or something, and decided to take the king guy hostage. To do this, we tried to infiltrate the heavily guarded castle. Due to a combination of crit fails and crit successes on our rogue's part for disguise checks, I ended up disguised perfectly as... a pretty princess. We decided to roll with it, and pretended to be visiting dignitaries. Or I did, anyway. The rogue, with our druid wild shaped into a snake and curled around his wrist, snuck around to the servants' entrance. He disguised himself as a staff member with one of the uniforms that had been left nearby, crossdressing as a maid because that was the only uniform there that fit. He reached the king's chamber with a tray of food, and I navigated to the throne room by shouting anyone who tried to stop me into submission with my ridiculous Intimidate. The plan was for me to get him to come alone to his chambers, where the rogue and druid would be lying in wait to ambush him without interference from the guards. My character was currently disguised as a female, and his main example of a female was our druid. As such, he decided that the what a female would do to get the king to his chambers was promising sex while riding giant birds. Somehow, it worked. The plan went off without a hitch. We had the king in our custody, leaving us free to... strip him, apparently. The first thing our rogue decided to do was take all the king's clothes (to sell). Then, once we had used our hostage to gain control of the city, he took all the guards' clothes (so they couldn't use their gear to rebel). Then, he decided that using a castle as our fortress of evil would be too obvious and we should go set up in a random cave system instead.

A rather slow, and suruptitious way of doing it. But, my character created several shell companies, and then slowly bought the entire town with proceeds from loot and the Navigator's family. Not glammourious, or flash, or anything. Just, good business.

In a recent campaign we were exploring this rather mad country and the prime minister was an insane, incredibly powerful wizard. We obliviously broke into his mansion after getting a suspicious castle mentioned as what was SUPPOSED to be flavor text; rather than follow the rails; so the DM had us fight him like 10 levels early.

The actual battle is kind of a long story; but ANYWAY, it turned out that the guy had instituted a policy of ritual cannibalism and eating the prime minister makes you the new prime minister, which I had done. The world was shortly to be destroyed, though, so the party installed a magical talking penguin as regent instead while we left to stop that.

Wait, sorry, you said WANTED TO, nevermind.