I believe based on your question that the taking of Conwy castle during Owain Glyndŵr’s rebellion against the English would be a perfect example of a castle being taken unconventionally.

Before I go over the actual details I’ll give some background since I’m assuming he isn’t known outside of Wales: Owain Glyndŵr is something of folk hero in modern day Wales being dubbed by some as the ‘Last True Prince of Wales’ since he was also the last Welshman to hold the title. His uprising began in 1400 and ended in 1415 during which time he fought a guerrilla campaign against the English monarchy in an attempt to gain Welsh independence after years of poor treatment of Welsh people including them being excluded from living inside the walls of the towns and cities of Wales in favour of English settlers.

One of the major ways that attempts at rebellion was put down was the ‘Iron Ring’, a series of fortifications surrounding the north of Wales including such examples as Caernarfon, Harlech and Conwy castles. There is an often repeated fact that Wales has more castles per square mile than any other country in the world and a large reason for that was the importance placed on castles by English nobles to further the suppression of rebellions in Wales.

Now on to the actual taking of Conwy castle, the event that first threw the Tudor dynasty into notoriety. Conwy was known as an almost impenetrable castle, it’s position made the building of siege engines difficult, the castle has eight towers and high walls. So the only way to take the castle would have been either a protracted siege or some kind of trickery. Since at this point the Welsh didn’t have the forces or resources for a protracted siege that was out of the question. But instead on Good Friday, April 1st 1401 the garrison of Conwy left to go to a church service leaving the castle undermanned, supposedly guarded by just two gatekeepers. It was during the service that the castle’s carpenter and a few of his apprentices approached the castle and requested access to complete building work that had to be, they claimed, finished urgently.

The gatekeepers allowed them entry and soon found themselves being set upon and stabbed by the group who then let in Gwilim ap Tudor with forty followers who quickly barred the gates and locked the garrison out. The garrison would only realise what had happened after returning from the parish and seeing the new occupiers of the castle atop its walls.

The group would then quickly be besieged by Henry Hotspur who also brought with him the first cannon to be used in Wales. The castle would only hold out until June 24th where the castle was surrendered and Gwilim and 4/5’s of his men were allowed to leave.

Whilst the castle wasn’t held for long it’s capture was still a huge propaganda victory for Owain and the fact that such an impressive fortress fell into Welsh hands spurred on this early rebellion and made the hope for independence seem a little more likely.

In my era "some sort of betrayal" was not an unconventional way to lose a city or similar besieged place. Even discounting ones that the commander ceded very early on, there were enough occasions where unhappiness in the defender ranks fatally undermined the defence. While Shen Pei facing three such betrayals (defeating two of them, falling to the last) in the defence of Ye was unusual in the scale, a defending officer fearing for the future or unhappy with the commander (a siege perhaps not always the best place for keeping happy relations between superior and subordinate) was not unusual.

Both my stories come in 215 CE with China plunged into civil war.

Bluff

Tensions between Sun Quan in the south and Liu Bei, the junior ally who had recently taken the province of Yi in the west, exploded. Or perhaps more accurately, got a bit more heated. Sun Quan claimed the deal overseen by his commander Lu Su, a long term planner who sought grand things for his master, in 210 was a loan of Nan commandery and time to give the entire province to Sun Quan. Liu Bei had other concerns at the time, settling his new lands and the attack of Cao Cao on nearby Hanzhong, so he sought a delay of any attempts to implement Sun Quan's version of the agreement "agreement" (though his faction did not believe the deal in 2010 was a loan) until a time better suited. Sun Quan sent officials anyway and predictably Liu Bei's man in Jing, a man famed for his beard, honour and warrior skill, drove them off. Sun Quan ordered the attack with Guan Yu likely undermanned with resources and soldiers having been pulled away for the invasion of Yi.

Lu Su remained at Lukuo to pin Guan Yu at Gongan and prevent him from being able to move to reinforce the defences elsewhere. Lu Meng, a bold warrior turned into a tactician after Sun Quan personally pushed for him to study, was sent with 20,000 men against the southern commandries of Changsha, Guiyang and Lingling.

The first two areas surrendered on getting Lu Meng's letter, the local forces not having much hope of facing that invasion force but Hao Pu at Lingling decided to hold out at his capital of Quanling. This wasn't too much of a problem as Lu Su and Guan Yu negotiated but when Liu Bei arrived at Gongan with an army, things changed. Sun Quan had led his own forces to Lukuo so both warlords faces off but Guan Yu was now free to move. As Guan Yu advanced down south, Lu Su moved to Yiyang in Changsha to block him. Lu Su had only 10,000 men and needed Lu Meng's 20,000 men to reinforce him so issued a summons while Lu Su engaged in diplomacy to buy time.

Lu Meng got the letter but decided one last trick, telling everyone in camp he was going to storm Quanling the next day. He sent a friend of Hao Pu, Deng Xuanzhi, to Hao Pu warning that Liu Bei was in Hanzhong fighting desperately, Guan Yu was held by Sun Quan so no help was coming while reinforcements were streaming in for Lu Meng. The Sun commander admired Hao Pu's sense of honour of course but it was only right to warn him that help was not coming, that Hao Pu simply had not heard the news. Fighting on would only lead to death and potential execution for Hao Pu's elderly mother.

Hao Pu surrendered unsurprisingly. When Hao Pu came out of the city to meet Lu Meng, the latter sent soldiers to secure the gates while Lu Meng showed Hao Pu due courtesy and personally escorted him to a boat waiting for the surrendered officer. There he then showed Hao Pu the message of recall and Lu Meng, perhaps not entirely kindly, laughed as Hao Pu read it and became increasingly mortified.

Lu Meng kept a force under Sun Jiao in Lingling then went north to Lu Su having strengthened their hand. Neither side wanted a long war with the far larger power of Cao Cao meaning they were wary of getting bogged down so an agreement was made, Lingling returned to Guan Yu but Guiyang, Changsha went to Sun Quan and the Xiang river became the border. An uneasy peace could be maintained and focus turned elsewhere.

Lu Su would die in two years of illness, Lu Meng became his successor as Sun Quan's main commander and in 219 seized control of all of Jing in a swift attack, Guan Yu (who had been leading an invasion against Cao Caos general Cao Ren at the time) was executed and the balance of power between the allies permanently shifted in Sun favour. Lu Meng would die soon after of illness, perhaps having led his final campaign while badly sick. Hao Pu, who perhaps was not the best judge of honesty, would become a minister in Sun Quan's court but committed suicide in 230 for his backing of Yin Fan, a spy from a rival (Cao Rui) court.

Getting Lost

To the east of Liu Bei's position was the mountain passes of Hanzhong and the theocratic regime of Zhang Lu of the Five Pecks of Rice movement. The controller of the Han and the most powerful warlord Duke Cao Cao decided to attack in the spring but his attempt to take an unexpected route through the Lianyun Road but was disrupted when he then first had to fight against the Di people under king Doumao and it took till autumn for him to reach Zhang Lu's position.

Zhang Lu was said to want to surrender but his brother Zhang Wei had assembled ten thousand men and fortified Yangping Pass. Cao Cao meanwhile was expecting an easy fight as locals had told him the pass was indefensible, he was not too pleased to discover how difficult it would be to take. The territory was mountainous, the roads narrow, winding and sometimes plank roads over gorges which, though showing skilled engineering, was not ideal for manoeuvring a large army or supplies.

According to senior adviser Dong Zhao's account of events, Cao Cao tried an attack on some of the camps in the hill but it failed with heavy losses while his supplies may have been stretched having had to spend the summer months fighting to get here. Cao Cao gave the order to pull back despite opposition from some of his officers, sending his trusted kinsman Xiahou Dun and his loyal bodyguard Xu Chu to do so. However, as it was getting dark, some of the troops from the front got themselves lost and found themselves at one of Zhang Wei's guard positions. Fair to say, neither side were expecting that.

The defenders there panicked and fled, the very surprised attackers were now inside Zhang Wei's position and got word via some of the officers back to Xiahou Dun. There was disbelief among the generals but Xiahou Dun went forward to check this barely believable tale and on seeing this was true and that defenders in one of the camps were fleeing, sent word to Cao Cao. He pressed the attack on the confused defenders, the defender's position collapsed during the night, Zhang Wei was killed as he fled.

Zhang Lu abandoned his capital Nanzheng but left things intact as a show of goodwill and of loyalty to the central government. He withdrew to the support of the local tribes in Ba, from there he would carefully negotiate a favourable surrender. Was made a General and ennobled as were all his sons, his daughter married into the Cao family and was moved to Cao Cao's stronghold in Ye. Zhang Lu died a year in and is a patriarch of the modern Taoist Church.

Cao Cao's grip on Hanzhong would last only four years, conquered by Liu Bei with kinsman Xiahou Yuan slain in battle (and his young son Rong dying despite the best efforts to carry him away), becoming a key defensive point for the Shu-Han regime of Liu Bei and his son Shan.

Cao Cao would rise to King before his death on the 15th March 220 and by the end of that year, his eldest living son Cao Pi became the founding Emperor of Wei with the abdication of the Han Emperor Xian. Xiahou Dun died of natural causes in 220 Cao Cao, Xu Chu was in great distress after Cao Cao's death but would receive honours from two Wei Emperors till his death in 230. Dong Zhao whose memorial chronicles some of these events would serve Cao Cao's grandson Cao Rui and became an Excellency before his death at the age of 82.

Hope this helped or at least was fun

Sources

Records of the three kingdoms by Chen Shou, annotations by Pei Songzhi, translated by Yang Zhengyuan

Comprehensive Mirror in Aid of Governance by Sima Guang, translated by Rafe De Crespigny

Imperial Warlord: A Biography of Cao Cao 155-220 AD by Rafe De Crespigny

Generals of the South: The Foundation and Early History of the Three Kingdoms State of Wu by Rafe De Crespigny