Prior to 1235, the Polovtsy nomads of the Russian steppes north of the Black Sea had blocked the Mongol's westward expansion. The destruction of the Polovtsy nomads by Christian Russian Principality crusaders wishing to convert these 'heathens' eliminated the western barrier for the Mongols. In 1235 the Mongols began their push westward into Russia, sacking Vladimir and destroying Kiev. That winter the Mongol Horde destroyed the Russian principalities, sacking city after city in quick succession, then rode into central Europe.

In April of 1241, under the leadership of the Khan Batu (Batya), the Mongol army simultaneously attacked Poland and Hungary, setting up a permanent base in the latter. The following year when word was sent of the death of the Great Khan Ogedei, Batu withdrew his armies southwest out of Hungary and swung them south along the Adriatic Sea, east across the Macedonian peninsula, and finally northeast across the Danube River. Their withdrawal toward Saray (Sarai) on the northwest shore of the Caspian Sea took 9 weeks to march the army of 370,000 men and auxiliaries. Along their way, of course, Batu's armies still required prodigious amounts of food and supplies to sustain them. Unknown to the residents of Castle Tîrgu, the massive armies of the Mongol Horde were bearing down directly toward them.

The Horde had been traveling for 6 weeks, their provisions were getting low; advanced scouts located the Tîrgu area and sent a message back of a location to replenish their stores. The defenses of Tîrgu were standing in the way of the Horde's provisioning.

The bulk of the army arrived and debouched on the plain south of the castle. As they had no advance warning, the inhabitants of the castle had little time to prepare.

The mounted archers, numbering over 40,000 strong, lined up a dozen deep in a wide arch along the southern side of the castle and fired a recorded 10 arrow volleys at and over the castle walls. Within a matter of minutes, half of the defenders in the castle were dead. There are a number of accounts of scores of defenders killed in the first volley, and some of these were concealed behind arrow loops, which usually afforded complete cover from small missile attacks. These arrow loops were overwhelmed with the sheer volume of missiles. One written account from a serjeant's report from the attack is that of an arrow loop being so bombarded as to plug the slit with arrows, rendering it useless to the defenders. The written account states the plug was rammed at with an iron rod in a vain attempt to clear the jam.

The castle's defenders surrendered after the initial missile attack. King Mendax IV received a messenger from Batu and capitulated to the Golden Horde leader's demands in exchange of the lives of the remaining defenders and the wounded. Batu agreed, took the king and the royal family prisoner and the remaining defenders and wounded left within the day.

The Mongols sacked the castle and raided its large provisions. The Great South Wall was sapped, which collapsed in multiple places, as was the Inner Bailey. The interior of the castle, including the Cathedral, the Great Hall, and he opulent Royal Halls was set alight after being stripped of anything valuable.

Afterwards the castle, as a defensive structure, was abandoned. The remaining walls being raided for stone from local towns or left to collapse.

For a time, the Tîrgu area was under domination of Saray and began to prosper again under the pax mongolica. This allowed merchants from the east to trade freely into Eastern Europe, which developed into the Northern Silk Road.

For the next century or so, farmers used the castle site as a small marketplace around which a large town grew. Aided by a century of peace from the pax mongolica, the area was starting to rebuild into an region of prosperity again. Merchants from the Far East started to bring their goods to market in Tîrgu.

In December of 1347 the merchants brought more than trade goods with them; a few townspeople started showing signs of strange black buboes on their skin. The Black Death had come to Tîrgu. Within the winter, the local population was devastated.

Any remaining survivors fled and for the next 5 centuries the ruins fell into the state of oblivion seen today after 600 years of neglect.