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In a civilization with modern to near-future technology, are there ways so that a siege against fortresses, and eventually against cities itself, would still be required to properly conquer said city, or even be the most effective method? Are there ways to reduce the effectiveness of tactics in this setting that do not involve soldiers actively conducting siege warfare similar to the medieval era?

Siege is still an element of modern warfare. If a faction wants to get in a city and another one doesn't want to let them in, a siege is the natural consequence.

We have had some famous examples in the recent years, just to cite a couple:

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  The battle of Stalingrad
  
  

  
  

  The Battle of Stalingrad (23 August 1942 – 2 February 1943) was the largest confrontation of World War II, in which Germany and its allies fought the Soviet Union for control of the city of Stalingrad (now Volgograd) in Southern Russia.

  
  

  
  


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  Siege of Leningrad
  
  

  
  

  The siege of Leningrad [...] started on 8 September 1941, when the Wehrmacht severed the last road to the city. Although Soviet forces managed to open a narrow land corridor to the city on 18 January 1943, the Red Army did not lift the siege until 27 January 1944, 872 days after it began. The blockade became one of the longest and most destructive sieges in history, and possibly the costliest in casualties suffered.

  
  

  
  


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  Siege of Sarajevo
  
  

  
  

  The Siege of Sarajevo was the siege of the capital of Bosnia and Herzegovina, and the longest of a capital city in the history of modern warfare. After being initially besieged by the forces of the Yugoslav People's Army, Sarajevo was besieged by the Army of Republika Srpska from 5 April 1992 to 29 February 1996 (1,425 days) during the Bosnian War. The siege lasted three times longer than the Battle of Stalingrad and more than a year longer than the Siege of Leningrad.

  
  

  
  

As additional info on what is so difficult with siege, look at the siege of Montecassino: as long as the abbey was a no fight zone, German troops were stationed outside, and were a relatively easy target. Once the abbey was bombed and became a ruin, it became a wonderful hiding place for the German troops, who could hide and attack with much more ease.

In principle, a siege is a strategy with the express goal of defeating your opponent through slowly attritting their forces.

With modern warfare’s application of combined arms — the tactical use of land, air, and naval units simultaneously to achieve a tactical goal — the pace of conflict is so fast that battles are either decisive and fast or forces keep their distant from each other.

But, Sieges are still relevant today in conflicts involving cities with civilian populations where the attacking force wants to take the city more or less intact without killing or at least minimizing the civilian population.

If the defending forces value the civilian population, they might abandon the city knowing the attacking units aren’t going to harm the civilians left behind. So no siege since the defenders would leave before they were encircled. The Russian invasion of Georgia can be interpreted as an example of this situation

If the defending forces don’t value the civilian population, then they can use them as human shields or as hostages. This causes the attacking forces to move slowly, house by house and street by street. The recent news showed this kind of warfare in the The Siege of Aleppo, and other battles in the Syrian.

I guess the same arguments apply to fortresses. But, unless a fortified area was needed to be kept intact, I would think attackers would destroy it with air power.

Judging from recent history sieging is a problem. Yugoslavian war had almost 4 years long siege of Sarajevo.

Siege for Aleppo in Syria took time between 2011 and 2016 and although government stated that they control whole city there are still some fights on the outskirts.

Main problem with sieging is that you might want something from that particular place. It might have industry, control over port, banks or information, technology or the place is good for hiding and need to be control for defence purposes (like Tora Bora).

With modern and near-future technology you could limit the amount of soldiers. Self-driving drones, artillery that could be operated with limited staff. But you would still need some foot on the ground as the everchanging landscape of sieged city/fortress would require human-like abilities to recognize, adapt and react.

Presuming the attacking force has modern warfare of even today, a siege requires that there be some reason the attackers don't want to just obliterate the defenders. Or that they are having a hard time doing that.

If the attackers simply want to wipe out the defenders they can do this in short order. They can drop a huge variety of chemical explosives from fuel air weapons to bunker busters. Any ordinary building with ordinary walls, doors, windows, etc., will go to the fuel air weapon. Any hard target with walls less than 6 meters thick will fall to a bunker buster. Even if the defenders are not killed outright, they will be buried under many meters of rubble.

If it's some extreme situation there is even the possibility of going nuclear. It would probably only happen if the attackers were somehow a coalition of nearly every nation in the world, and the defenders somehow drastically offensive to nearly everybody in the world. Offhand I can't think of a candidate. But it could happen. As a science fiction angle, maybe it's invading aliens and they have vile habits.

There must be some reason the attackers want to be relatively selective.

Perhaps there is a civilian population that is relatively uninvolved, and the attackers don't want to appear to be monsters for killing them wholesale. For example, the intended targets might be some small group of individuals that would be perceived as legitimate targets. Maybe terrorists launching missiles from a suburb. Or a defeated leadership after a war could have retreated to some stronghold in the middle of an otherwise pacified civilian population. There is desire to get the targets but not wipe out large numbers of non-combatant civilians.

Perhaps there is some value in the defense site such as historical monuments or buildings, famous works of art. Maybe it's a museum with many thousand works of art. Maybe it's a famed base of a religion, such as a major church or shrine.

Maybe the defenders have some other resource the attackers don't want to simply wipe out. The only son of the president of the attackers, for example.

Maybe the defenders have some suicide option. They have a bunch of bombs placed at key locations around the world, and if they get smashed to little bits the hidden bombs get set off.

Or, to go all science-fiction on you (since you added that tag) the defendants might have some counter measure.

Maybe they've got extra hardened bunkers. Perhaps they've got the local equivalent of a really good metal smith, and their bunkers simply shrug off the bombs. Or maybe they have some super tunneling ability, and they can hide 100 meters under the ground and pop up 2 km from the bombed location, take some shots, and hide again.

Maybe they've got some really good anti-aircraft gear. Maybe two or three decades of research on the Iron Dome has resulted in something that makes it very hard to hit a protected target with planes or missiles. Though in the quite near future that could probably be pretty much brushed aside. Project Thor would see orbital kinetic weapons hitting the target at mach 10. It's quite a challenge to know what would stop that. It's unlikely that even a laser based defense could stop that, at least with next-couple-decades tech.

As to methods of siege, modern tech has provided lots of options. Depending on the nature of the target. Of course there will be some kind of surrounding to prevent additional resources getting in. There will be attacks on command and control such as electronics, radio equipment, etc. There will be attacks on any vehicles. There will be attempts at infiltration, to get intel, to open defenses, to perform sabotage, and to attempt to convince the defenders to give up.

We have also seen quite a few innovations recently. Playing loud music 24 hours a day to keep the defenders from sleeping. Chemical irritants that nudge right up to but don't cross limitations on chemical warfare. For example, dropping canisters of rotten egg gas. Bright search lights and laser beams shinning into the defender areas, both to obscure what the attackers are doing and just to be irritating. Radio noise to keep them from communicating. Enough electronic noise can even stop commercial grade computers from operating.

And depending on the nature of the defenders there may well be lots of other leverage. Do the defenders have relatives outside the defense location? Do they care about some religious site? Do they have financial holdings they are hoping to use after they get away from the siege? All of these could be threatened to good psychological advantage.