Many Ukrainians have signed up to territorial defence units and are preparing to join the fight as every house, every street, every checkpoint gets ready to resist the Russian forces.
In antiquity and the Middle Ages, armies would lay siege to cities but fighting usually took place at their fortifications and populations were often starved into submission.
After this, battles were conducted in open spaces between armies, rather than in inhabited areas.
The mother of such battles was the fight for the Russian city of Stalingrad (now Volgograd) in 1942-43 between Soviet and Nazi forces, which left an estimated two million people dead.
Many Ukrainians have signed up to territorial defence units and are preparing to join the fight. Every house, every street, every check point will resist, until death if necessary," the mayor of Kyiv, former boxing champion Vitali Klitschko, wrote on Instagram.
The next step in Russia's invasion of Ukraine could lead to urban warfare, an immense challenge for even numerically superior armies with deadly consequences for civilians caught up in the fighting, experts say.
Russian artillery and rockets have been striking cities including the capital Kyiv, as well as smaller regional centres such as Kharkiv, Mariupol and Chernihiv.
Inhabitants there, as well as in the southern port of Odessa, another strategic target, are now preparing for possible ground attacks.
There remain doubts about whether Russia has the resources needed to capture and hold urban areas in Ukraine, and whether the country and President Vladimir Putin are ready to accept mounting casualties.
Defenders can also benefit from the height advantages from local buildings, with Russian tanks and other armoured vehicles vulnerable to attacks from above.
Between 2016-17, the Iraqi army required eight months to dislodge a few thousand jihadists from the Islamic State group in Mosul after they had seized control of the city. Having failed to make a decisive advance in the early phase of its Ukraine campaign, the Russian army has already been facing a thaw that could make progression on key routes problematic due to mud.
In 1812, the retreat by French emperor Napoleon's troops could not be completed in time before the harsh winter because it was severely slowed down by mud.
More than a century later, the heavy Ukrainian soil got in the way of Hitler's march on Moscow, and later of Russia's counter-offensive of 1943.
Now, Russians are knocking on the gates of Kyiv. The capital's northwestern edge has witnessed more than a week of ferocious bombardments that have levelled parts of towns.
Tens of thousands have braved near-constant shelling to flee to the relative safety of the Ukrainian capital from working-class towns such as Bucha and Irpin even as US and its NATO allies have stepped up weapons deliveries to Ukraine in a bid to repel the Russian advance.
However, the US-led defence alliance has refused to send in warplanes due to fears that this could trigger an even bigger war between Russia and the West.
Local knowledge, mobility and early occupation of vantage points typically favour defending forces in urban combat.
A war monitor said Russia had drawn up lists of 40,000 fighters from Syrian army and allied militia ranks to be put on standby for deployment in Ukraine.
According to a senior US official, the Ukrainians have already received some 17,000 anti-tank weapons from various Western countries, including several hundred Javelin launchers, and Washington announced on Saturday new military aid totalling $200 million and including the anti-tank weapon.
Ukrainian forces have also received thousands of other anti-tank weapons, including British NLAWs, Swedish-made AT4s and Carl-Gustavs, German Panzerfausts and Spanish Instalaza C90s making the task of Putin's army that much harder.
US-made Javelin that has become the weapon of choice, to the point that a meme circulating on social media has transformed it into a religious icon borne by Mary Magdalene, an emblematic saint of the Orthodox church.
Dubbed "Saint Javelin," protector of Ukraine, she brandishes one of the missiles under a halo in the yellow and blue of the Ukrainian flag.
Russia invaded Ukraine in the early hours of February 24, setting off the worst conflict in Europe in decades. Over a month into the invasion, Moscow has faced military losses and unprecedented sanctions in exchange for relatively meagre progress on the ground.
The US government says recent days have brought more intense aerial and naval bombardment as Ukrainian resistance holds up the invaders' advance on land.
Attacking forces remain 15 kilometres from the Ukrainian capital Kyiv to the north-west and 30 kilometres away to the east, able only to bombard it from a distance.
Capturing Kyiv had appeared the Russians' top target as they entered the country on February 24, hoping to topple President Volodymyr Zelensky's administration.
But despite mobilising a force of between 150,000 and 200,000 troops, Moscow failed to anticipate anything other than weak resistance in Ukraine.
The number of soldiers killed cannot be verified, although the Pentagon estimates as many as 7,000 Russian dead in the first month, according to the New York Times.
That would amount to more losses than the US suffered in the years-long occupations of Iraq and Afghanistan combined.
"Putin's offensive is stuck despite all the destruction that it is bringing day after day," German Chancellor Olaf Scholz said.
For its part, Kyiv acknowledged 1,300 troops killed in action on March 12. With their forces bogged down, the Russians have transformed their offensive into a string of sieges apparently aimed at wearing down and demoralising the Ukrainian population.
Around 10 million people have left their homes, while the UN human rights chief says almost 3.6 million have quit the country.
Throughout the offensive, Russia has only captured one major urban centre -- Kherson in south Ukraine. Mariupol to the east remains subjected to a siege that has caused international outrage.
Uncaptured northern cities like Kharkiv, Ukraine's second-largest urban centre, are under constant bombardment, while pressure is also growing on those in the south and east.
Mykolaiv, which blocks the way along the Black Sea coast towards Odessa, is also under artillery fire.
However, it remains to be seen whether Russian forces will enter Kyiv even as Zelensky's government has openly declared that it is ready to take on Russian forces in fortress Kyiv dotted with defensive positions, checkposts and soldiers ready to fight the Russian forces till the last.