Mobile phones are banned and punishments are draconian, yet locals enjoy certain benefits under the Islamists' rule.
"I fled Mosul when Isis threatened to conscript my brother as one of its fighters, though he is under 18 years of age," Ali Hussein Mustafa, a student who left the city a week ago, said.
The self-styled "Islamic State" is seeking to bolster its military forces as it wages war on many fronts and it has introduced a new rule under which men under the age of 18 are no longer exempt from conscription.
The Iraqi government is threatening that it will soon send its army north to recapture Mosul, a city of two million, the loss of which last June was the first in a string of victories by Isis.
The Iraqi Prime Minister Haider al-Abadi announced this week in an interview that "we are now planning an offensive against Mosul in a few months".
If the army does attack it will face formidable resistance from the armed forces of Isis that may now number well over 100,000 in Iraq and Syria. Moreover, people in Mosul, the northern capital of Iraq, are divided in their loyalties, judging by interviews with The Independent conducted this month, either after they left the city or by mobile phone, although Isis has banned their use.
In a predominantly Sunni Arab city, many are more frightened of largely Shia Iraqi government forces than they are of those on the side of Isis, though they may not like either.
"Some fighters treat the residents cruelly and harshly, while others are well-educated and treat the people well," Ali said. He cites a local mathematics teacher who joined Isis recently "but was very kind to people and gave money and food to the poor. He often asked me whether I have any information about widows and the disabled in the city. He was donating part of his salary to them."
Though Ali and his family have become refugees he still argues that many Isis fighters are better than their equivalents in the Iraqi army, which held the city for 10 years before 2014.
At the same time, Ali recalls examples of extreme barbarity, with the hands of men accused of theft being publicly amputated and people discovered using mobile phones receiving 30 lashes.
Isis is fearful of spies using mobile phones relaying information to US drones that hover continuously overhead. There are daily air strikes by US aircraft, though most of these are taking place outside the city. Several senior Isis officials are reported to have died when their vehicles were targeted.
Foreign fighters are particularly brutal towards women not wearing the niqab, a piece of cloth covering the head and face. Ahmad, a shopkeeper who still lives in Mosul, said he was shocked when a woman he knew was taken to a local police station because her eyes were showing even though she was wearing a niqab. He said her punishment was that "a bit used by donkey was put in her mouth and she was told to bite down hard on it - which she did and then had to be taken to hospital afterwards because she was bleeding heavily."
Mosul is increasingly isolated from the outside world because of the prohibition on the use of mobile phones. Isis has blown up many towers that previously carried a signal, though mobile phone use is still sometimes possible from high places such as rooftops or hill tops.
One place previously used was a stage in Concerts Square in al-Majmu'ah al-Thaqfiyah area but three people were whipped for making calls from there. Whipping is also the punishment for those found at checkpoints to have SIM cards in their pockets.
There is an increasing number of checkpoints inside the city and those at the main exit points often stop anybody leaving who does not have a valid excuse. Trenches have been dug to stop Kurdish Peshmerga forces to the north and east of the city - with, in one case, Isis even putting out a public tender for a trench system.
The Kurds have made advances in recapturing much of the Sinjar area west of Mosul, advancing behind heavy US air attacks against any point where Isis is resisting. But this tactic would be less feasible in built-up areas such as Tal Afar or Mosul itself.
Kurdish leaders say they would not advance into Sunni Arab areas where all the Sunni would rally against them. One Kurdish commentator, Kamran Karadaghi, said Kurdish public opinion would not welcome a battle for Mosul in which there would be heavy losses. He says people would ask: "Why should so many Kurds die for a Sunni Arab city?"
Despite Mr Abadi's declaration that the Iraqi army will recapture Mosul this year, such an assault appears to be well beyond the strength of the Baghdad government, if it relies on its own regular army. This is now said to number 12 brigades with a nominal strength of 48,000 that might be made battle-worthy when aided by US advisers.
But this is barely enough to defend Baghdad and fight in some neighbouring provinces, while the disintegration of the Iraqi army last year as it abandoned northern and western Iraq is not a hopeful portent.
In the past, Iraqi officers have always bought their jobs in order to make money through embezzling funds intended for supplies of food and equipment or by levying tolls on all goods vehicles passing through their checkpoints. Mr Abadi revealed last year that 50,000 soldiers in the army are "ghosts" who never existed but whose salaries went to officials and officers.
The most effective armed force of the Iraqi government is made up of Shia militias which have retaken Diyala province north-east of Baghdad and Sunni towns to the south of the capital. But the Shia militias are highly sectarian, killing or driving out Sunni Arabs who are treated as supporters of Isis whatever their real sympathies.
Isis has targeted Shia civilians in Baghdad and elsewhere using car bombs and suicide bombers causing horrific casualties, thus enabling Isis to pose as defenders of the Sunni Arab community when the Shia retaliate.
Life in Mosul may be grim for its inhabitants with shortages of clean water, fuel and electricity, but food supplies are still adequate. In some respects Isis runs a more active state apparatus than Baghdad, which has traditionally done little for the victims of violence.
Ali Hussein Mustafa said that when there was fighting recently between Isis and the Peshmerga, many of the Sunni Arabs from Tal Afar fled the rocket and artillery fire and went to Mosul where Isis organised their accommodation.
Isis can afford such bounty because it has confiscated the houses of Christians and others who have been forced to flee. A successful counter-offensive against Isis leading to the recapture of Mosul does not look likely this year whatever Mr Abadi's declared intentions. Many of those in the territories of the "Islamic State" would like to end its rule, but only if it were replaced by an Iraqi army that is disciplined and non-sectarian enough to provide an acceptable alternative.
Patrick Cockburn is the author of 'The Rise of Islamic State: Isis and the new Sunni Revolution'