The most wanted man in Aleppo is feeling satisfied. Less than a week before, he had helped pack the last of 25 tonnes of explosives into a tunnel dug under a hotel filled with Syrian troops.

“I was sitting in this room,” said Abu Assad, the rebel commander of Aleppo’s tunnel forces at another of the city’s frontlines. Smiling, he cupped his ear and added: “We heard the blast from here. It made us very happy.”

The explosion that destroyed the Carlton Citadel hotel in central Aleppo on 8 May rumbled well beyond the 9 miles (15km) between the bomb site and where the commander now sat recalling that day. Its destructive force sent shock waves through the well-dug-in Syrian military command in the city’s until-then impregnable west, and jolted to life an opposition whose war had been going badly.

The giant plumes of dirt and rubble that ballooned several hundred metres skyward as the hotel crumbled, killing 30-50 Syrian troops, have fast become one of the most jaw-dropping images of Syria’s civil war. In three years of aerial blitzes, scud missiles, chemical attacks and suicide bombs, little else had been captured so spectacularly and in real time (video).

“That was one of the best things about the operation,” said Assad. “The effect on morale was immediate. Ever since, the men have wanted to fight more than before. We have called this “Operation Aleppo Earthquake”.

He said the Carlton was being used as a barracks by Syrian police officers and by paramilitaries known as shabiha. Syrian officials angrily denounced the attack and the tunnel bombs used by the rebels, claiming they were killing and maiming indiscriminately and destroying the city’s soul and identity.

Flush with the success of the bomb, Assad chose to reveal himself to the Guardian as the leader of Aleppo’s tunnellers. He claimed not to be bothered by the fact that showing his face would give an already furious military even more reason to hunt him down. “I want them to be scared of me,” he said. “They need to know I am coming for them.”

The band of underground rebels, around 100-strong, are credited with doing more in less time than any other group in the 38-month war to take the fight to an ascendant military who, with the help of their backers, have clawed back lost towns and cities over the past year.

To make gains above ground, the tunnellers have burrowed beneath it at least nine times in the past six months alone. “Every strange noise, every shift in the ground, they will think it is us,” said Assad, adding that he was given the idea to start digging by a Palestinian who visited him in northern Syria last year.

“They said they had some success in Palestine, so I decided to try it. It wasn’t hard to find the explosives. I have personally supervised nine tunnels.”

Nor was it difficult to find volunteers prepared to dig through rock and stone under the ancient heart of Aleppo, the arches and millennia-old homes and mosques which separate regime and opposition forces in some places by only a few dozen metres.

Assad, a rural carpenter before the war, said the hotel tunnel was 107 metres long and took 33 days to build. Other subterranean digs have stretched for 860 metres and needed many months. “We have more surprises for them, God willing,” he said. “They are taking a little longer.”

It says much about the unrelenting conflict that the only real shifts in a stagnant battlefield have come through reverting to bygone forms of warfare.

Chemicals used in the first world war, but not outlawed, have helped the regime secure Damascus. Scud missiles, a tyrant’s deterrent of choice a quarter of a century ago, have pulverised the north. And now tunnel bombs, used first by the Romans, then in the middle ages, followed by Ivan the Terrible, British forces on the western front, and Palestinian militants in Gaza, have made gains where conventional attacks have failed.

Medieval savagery has become synonymous with Syria, where more than 162,000 people have been killed since the insurrection morphed into open war, and close to half the country’s population has been displaced.

Assad fears that nothing will change soon in the north of the country, which now looms as a decisive landscape in the battle for who, if anyone, will eventually win.

“We thought very hard before we went for this type of war,” he said. “But we did not have any choice. We needed to do things this way to help us and to help the people.

“The first tunnel we dug was on the 20th day of Ramadan last year. It was only 17 metres and it did not take long. There were 11 soldiers and an especially brutal Alawite officer and there was no other way to get rid of them.”

That tunnel, like the eight others dug since by his unit, which is part of the main rebel unit in the north, Liwa al-Tawheed, carved a path through Aleppo’s historical core.

Both sides face off against each other near a Unesco-listed citadel that has stood firm through nearly 3,000 years of invasions and purges. This withering war poses perhaps the most serious threat yet to its formidable stone walls, which Assad’s men have been accused of threatening with their tunnel bombs.

From a vantage point near the tunnel entrance used to destroy the hotel, the citadel shows no signs of damage. Its walls soar defiantly from the edge of a no-man’s land around 200 metres away. Paper and rubbish swirl in eddies near the mound on which it stands. But not a soul moves through the abandoned buildings and masonry nearby. The Carlton hotel, though, is a pile of bricks.

The bombed site of the Carlton Citadel hotel, seen from a rebel position in the old city area of Aleppo. Photograph: Zac Baillie for the Guardian

“If they want to talk about the historical sites,” Abu Assad said of Syrian officials, “let’s look at what they have done. They torched the old marketplace [nearby]. Most of the old mosques have been damaged by them.

“We have done everything we could not to touch the important places. We recognise how important they are.”

A heavy weapon roared to life from inside the citadel, but the rebels standing guard on the frontline didn’t stir. Through plastic pipes which act as sniper positions, they had not seen a regime soldier in at least three months and the constant firing from inside the fortress posed little threat.

“We are on the front foot here,” said Omar Sarkan, an ageing fighter in a brown dishdasha (robe). “They can’t advance, but the tunnels are working.”

Back at the second frontline in an industrial area north-east of the city, Assad said he had a message for his enemy. “Is there still an army fighting for Bashar? Hasn’t the Syrian army delegated to Iranians, Iraqis and Lebanese [Hezbollah]? We have more of these bombs for whoever is on the other side. They will need more cemeteries.”

• This article was amended on 20 May 2014. It originally stated that tunnel bombs were first used in the middle ages, followed by the Romans. These should have been the other way around and have been corrected.