In this series I'm mulling, kaiju have been around since prehistory, turning up off and on right up until modern times. In the near-future of this alt-history, say in the next twenty years (2040ish), measures have been taken to minimize damage kaiju inflict on cities. In no particular order, here's stuff I've come up with:
Kaiju are always tracked via satellite that monitor all known/registered kaiju, 24/7; some kaiju are recognized via computer records, digital reconstructions of their appearance, but others are tracked the old-fashioned way, tagged like wildlife. Automated systems alert populated areas whether or not a kaiju is close by. Unregistered/new kaiju can almost always be detected by the same monitoring systems, due to size, ambient energy levels, radioactivity, etc. These systems may also carry nuclear payloads, railguns, laser cannons, etc., Star-Wars-space-program-ish kinds of tech on-board, in case defenses on the ground don't work.
Warning systems are in place in cities to keep people informed well ahead of time, in case an attack is imminent. There are regular kaiju forecasts like weather reports on TV, and warning systems on personal devices like you might receive an Amber alert, etc. Being as kaiju are an everyday part of human life in this alt-history, it's second nature for people to keep tabs on kaiju anyway.
The electrical grid is decidedly more diverse than it is now. The move to clean energy happened a long time ago, and basically any means to get it was okayed, depending on the need: solar, wind, geothermal, kinetic, etc. There are nuclear power plants, based on new improved tech (vaguely tokamak design), but there might be contention over it, since there are some kaiju in this universe that are drawn to nuclear energy. Although with these power plants, the "walk-away" functions allow for people to leave their stations immediately to evacuate, and ideally, leave little damage to the environment afterward. The point is to make the grid less centralized in case of attack, so the city doesn't suffer complete loss of power, but can continue functioning as if nothing has happened (especially underground, but I'm getting to that).
City defense systems begin several miles outside of city limits, in the wilderness or barely populated areas. Concealed [rail]gun turrets, for example, that pop up and fire upon approach by anything over a certain height or tonnage. Military bases, posts, garrisons, patrols, some military presence to keep an eye out. There might be other, less direct means, to minimize damage from potential combat: releasing pheromones, olfactory distractions, or maybe even geoengineering, massive land alterations that has terrain subconsciously leading kaiju away from city areas, etc. Still mulling that. Point being, these systems are almost purely deterrent, to push kaiju in the opposite direction, as a lot of man-made tech still can't outright kill them; the hope is to basically annoy them in another direction.
In the event a kaiju does push through this defensive line and is intent on getting into a city, there are measures in place there too. I hesitate to say cities are walled; and I don't know that I want to resort to straight-up sci-fi elements, like force fields, as I'm aiming for a certain level of suspension of disbelief; but it's possible there are such walls in place at least some of the time in certain parts of the world. There are also more automated and manned turret defenses, hoping to push the kaiju away at the last minute. There might be more to this layer of defense that just hasn't occurred to me yet.
People in the city must then evacuate certain burroughs/districts/etc. depending on which one is under greatest threat of being attacked. The means of doing so is decidedly less chaotic than in, say, Godzilla movies, where it's basically a churning panic. Almost all buildings feed into underground shelters, far below to accommodate for the immense weight of a kaiju pushing down on the ground overhead, and to just ensure it's not as simple as kaiju digging down to get them; these shelters are sprawling and city-like themselves, so much so that entire communities might just live there year-round.
Almost all buildings are also built with tech that was basically meant to protect against earthquakes but were since implemented for kaiju attacks. Specifically, magnetic systems that are being tested right now to keep taller structures from falling over during rumbling and shaking (I don't pretend to know really how it works, but it's one of many things I'm studying). Poaching an idea I believe I saw from NEON GENESIS EVANGELION, some buildings may straight up recede into the ground, although I don't know how sensible that is.
I'm also toying with the idea that there are defenses in some cities that are deliberately built with facades to look like skyscrapers, maybe as a means of distracting kaiju from other buildings and minimizing damage; once they get close to this fake building, any number of defenses could be activated...maybe it's wired to detonate and fall on a kaiju? Or it's a huge electrical conduit meant to electrocute a kaiju by siphoning all available power from the city into it for a concentrated attack? Maybe the facade falls away and reveals itself to be a tower of turrents? It could be one or all of these things.
It wouldn't be any fun without some sort of tokusatsu type of response, i.e. giant robots or something similar called in to take care of the threat. Once a kaiju is inside a city, it's always the desire of those in kaiju response to push it back out, as killing them rarely happens: so maybe calling a giant robot in to fight them is not the wisest course of action. They are, however, more precise than some military attacks, and definitely preferable to something like biological attacks or dropping a nuke. Their roles are probably almost exclusively meant to grapple a kaiju and get them out of the city; if they're going to fight, they'll do it where casualties are kept to a minimum.
I guess a note: military vehicles are now based on multidirectional technology, for increased mobility and control. Think Harrier tech for jets, or V-tol tech, but way more reliable. The idea being, the military may have retired a lot of the hardware that is only good at getting somewhere really fast but can't stay and fight, like jets that can deliver a payload but then have to bank, turn around, and hope to God they can fire again in time. I admittedly don't know how sensible that is, it's just something that made sense in the planning stages, but I'm not sure if it does in reality.
Also, military vehicles are probably dominantly drone tech. This is to minimize loss of life, as that's considerably more precious now in this alt-reality where the world population is drastically lower than it is right now on our world. So almost all response vehicles are unmanned. There is a human element, like pilots, gunners, troops on the ground, guys inside battle mechs, etc., but it's probably accounting for 10-20% of actual response forces dealing with a situation like this, and that's probably as a last-resort, or specialized tasks.
Similarly, most vehicles in the cities are automated. This is to prevent traffic that will clog up major roadways and keep people from getting to safety. Most shelters can be accessed a more convenient way, but in the event someone is in a car when an attack occurs, all vehicles take their occupants to a shelter entrance, disallowing them to leave until such a time the onboard AI can locate an entrance for them. There might be some ethical backlash to this that I need to think over.
The absolute last thing anyone in power or in charge of kaiju response wants in a kaiju vs. kaiju situation. This is why I hesitate to use defense measures like giant robots inside city limits. If two or more kaiju come into a city at the same time, it's absolutely imperative that they are not allowed to converge on one another. If they do, well, you can imagine.
And, finally, I guess, it's super important that a kaiju is not allowed to simply wander the city: attacks on it do not cease until it is again pushed outside the city limits. That way, on the off-chance it dies, there won't be a 20,000 ton corpse in the middle of a city that you can't move. That, and if the kaiju eventually feels like it isn't worth it to be in the city, it will ideally leave of its own accord, and wander back into the countryside. It's not the most noble sentiment, but then, at that point, it becomes some other city's problem.
So that's all I've come up with at the moment, lest I've forgotten something (I'm sure I have). I guess my question is:
Is what I have sufficient, and/or does it need more?
EDIT: Some notes on the kaiju themselves, I guess...
There are many varieties. Not necessarily species, just different types that aren't easily quantifiable. Mutations, aliens, indigenous creatures to Earth, cyborgs, robots...kaiju is more a catch-all term. They have varying degrees of power, capability, intelligence. Their desires, functions, drives, they're all relatively different. City defenses then have to cover a lot of bases in order to apply to as many hostile types or attacks as possible, as there's no telling what might appear. More specialized defenses have to be formulated on a case-by-case basis.
Their numbers compared to humans are very low. I'm about to indulge in slapdash math, which I hate, but it seems necessary to prove a point: being conservative, let's say this version of Earth only has about 2-4 billion people left; whereas there are only about 20-50 kaiju on Earth, though that number tends to fluctuate with new discoveries and arrivals. Kaiju then are drastically outnumbered, and the chances of being attacked by one aren't all that high. Given the amount of damage they can cause, though, their presence is treated very seriously and most big cities have to have these kinds of defenses in place.
Most do not or cannot reproduce; some are mutants and can't reproduce, some are refugees from other planets or dimensions, cut off from their own kind, etc. Some are long-lived, and have been around for centuries. Some go into hibernative or torpur states, so there can be months or years where they simply don't show up.
A grand majority cannot be killed by human weaponry; more often than not, they die of old age, or from fighting each other. Nuclear warheads can do the trick, but no one wants to use them. Experimental weaponry has its own concerns and baggage. Smaller varieties may eventually die from repeated military response: not all have the same level of strength or defense against our attempts to kill them.
Honestly, I haven't worked out the kinks on how kaiju fit into history yet. History doesn't fork significantly until the recent past, when a kaiju attacked a city and the world at large knew they were real. History has plenty of evidence that they existed before then, but there is a laundry-list of reasons why it wasn't taken seriously till this one dramatic event. Cover-ups, of course, but that's easy and cliche; there's more at work than that. Point being, I haven't gotten that far. It's not terribly important to the topic at the moment.
I will make more edits as necessary.