Well, if you want you could ask somebody else to control the defenders. Heck, I could do it if you want.

But anyways, concerning the city's strength there are a few things to take into account. The OP states that major cities are between 100,000 and 1,500,000 in population. Since Altearx is a remote fortress-city, it will probably be on the low end of that scale. Say we have 150,000 people. I'm not sure exactly what percentage of an early Renaissance era city would be soldiers, but if we just say 3% that gives us 4,500. 

Let's round that and say that there is a garrison of 5,000 highly trained soldiers, many of which will be in the form of mages. As the OP also explains, even those that aren't mages make liberal use of magic in whatever ways they can: augmented bodies, enchanted weapons and armor, and so on. In a fair fight, many of them could probably stand against a level one or two creature. It would take a few to fell anything more than that.

However, this will hardly be a fair fight. Altearx is a fortress city, and in medieval times it wasn't unheard of for 500 men to hold off a siege of 10,000. If you do go the siege route, the city will be a hard nut to crack because it would obviously have a ton of food stored inside and you would somehow have to stop ships from coming into the harbor with supplies. The fact that Elysium is in an odd phase between medieval and Renaissance times means that cannons might change things up a bit, particularly because they are quite good at blowing up walls. 

Since there are several thousand soldiers in addition to any forces in surrounding lands, militias, and unorganized civilian defenses, a siege is probably a terrible idea until Viktor has troops numbering above 10,000. A sneak attack is what would make the most sense. There will probably be several different barracks throughout the city, but your idea to bomb the main one would still severely cripple their defenses. Then, if you somehow managed to sneak inside the city walls, in the resulting chaos you could probably take over with just a few thousand creatures, especially if you have constructs and if some of those creatures are level 3 or 4.

Altearx really needs to be fleshed out more to talk about specifics, though. Ask questions like, "What type of magic could the sorcerers have? How could Viktor handle that?" and "Would it makes sense for the city to have tunnels below it, or sewers/floodgates that go to the harbor? If so, could Viktor's army get in through those? How would he find out about them?"