Rise of the Ratmen guide. This one is going to be frantic.
1) We're looking for a decent map layout.
We've got 4 outlying settlements, and these are the main spawn points of the ratmen swarms. Ideally want most of them on one side of your palace settlement. My winning run just now had one spawn SW and the other three N, NE, and E. This can help focus our defenses, while also giving comparatively safe areas for buildings like temples. Even better is if both starting trade posts are very distant from our market.
We also want our market defended. It looks like there will always be at least two sewers close to it, but we can restart until the other Palace settlement sewers aren't with it and the layout allows for buildings between the market and attackers. It is fine if the market is in the path of a swarm.
2) Hero choice:
Ratmen champions are strong, can't be charmed, and come in large packs, so I don't think Priestesses and Cultists are going to cut it. They aren't durable enough, fast enough, or have enough time to build up masses of bodies. Paladins are fast, strong, durable, and willing to go shopping when they have a minute and some cash. Let's go paladins. Lots of paladins. More paladins.
Get a few rangers early to help with the very early rats. They'll also push back the fog around our palace settlement so we can build. Rangers don't have the durability to deal with swarms, so we aren't making many; try to keep them alive, but don't worry about replacing them once a week or so has passed.
Slowly add some healers for free healing. As long as their guild isn't in the line of fire, we probably won't need to replace them. They'll gain levels on their own so their self-resurrection ability will take care of respawning them.
If you make a Rogue's Guild, then feel free to recruit a rogue or two to loot gravestones. There will be more than enough graves, which is free money.
We'll have guilds for monks, dwarves, and wizards, but we're not building any. Monks and Dwarves are too slow. Wizards would be incredibly lucky to make it to level 2 on this map.
3) Early tasks: We need to do several things right out of the gate, so crank the game speed down for a bit. Our first task is to place some buildings so the peasants have something to do while we're clicking elsewhere. We need a Warrior's Guild and Dauros Temple first, then place a new blacksmith and Agrela temple. Make sure you place the paladin pre-req's before the others so they get in the build queue first.
If the market is near the attack path of a swarm, then we want a Warrior's Guild directly between our market and their line of march. We want the Dauros temple, the Agrela temple, and our new blacksmith in an area of town that isn't in the direct way of incoming ratmen waves; none of them are priority targets for rats so they should be fine. A wizard guild will also be useful; I placed mine next to the market and warrior's guild, but somewhat off any lines of march. This helps buffer the market against incoming hordes.
Our second task is to get our market researching so we can upgrade it. We want level 3 immediately to take advantage of the half dozen or so trade carts that will spawn before the initial trade posts are overrun. One of my incoming trade carts carried over 900 gold, so it was worth over 2700 at the level 3 market, which means that one trade cart very nearly reimbursed me for getting the market to level 3. Your heroes need healing potions and rings of protection anyway, and you can add the amulet of teleport a bit later. The sooner your market is level 3, the more trade carts that get that 3x multiplier.
All of our outlying settlements are doomed. Our third task is to remove them from the tax routes and repair routes, after which we'll completey ignore them. We don't want our peasants or taxmen wasting time walking out to these settlements to die; they can die just fine in the Palace settlement while actually getting some work done. Even the gold generating buildings are worthless because basic Ratmen will steal it all and murder your approaching taxmen as an encore.
We're going to start recruiting paladins as soon as possible. We need a full Warrior's Guild of paladins by yesterday. The sooner they can buy potions and start gaining experience, the better.
We want to upgrade our new blacksmith early and often. Our paladins will have money to burn on the better gear. A level 3 blacksmith will also let us place a dwarven settlement somewhere to help fend off ratmen; I put mine next to the market where it could cover nearby sewers and be in the line of march for ratmen waves. You could also put it in an area that needs the extra defense. It can't handle any of the larger rat champion waves by itself, but a nearby Warrior's Guild or even just a few ballista towers go a long way.
4) Longer term and general tactics:
Whenever a broken sewer main crops up around your palace, put a small bounty on it. Don't worry about sewer mains in outlying settlements; it is dangerous for your heroes to go out there, and if they are out there then they aren't defending your settlement. Eventually paladins will raze the outliers when they have spare time. Build a few Rangers before placing a bounty on the sewer main that starts in your palace settlement so you have the might to deal with the champion that spawns when it is destroyed. Destroy it before Day 3 so rats don't spawn from it.
When we have enough gold, we're getting a second Warrior's Guild, again placed right in the line of march. Pick a different ratmen attack route than the first Warrior's Guild. Earlier is better so your new paladins can start getting experience.
Upgrade the Wizard Guild to level 3 as you have spare gold. Your paladins will get their gear enchanted, and you'll have access to Lightning Storm. A well aimed Lightning Storm can turn a large swarm of rat champions into a pile of grave markers. That said, the spell is expensive so mind your finances. If your paladins can deal with a swarm without you losing anything important, then let your paladins get the experience.
Don't be afraid to drop an inn directly in a ratmen attack path, especially while you are trying to build your paladin numbers. Inns are cheap distractions that can buy time for your paladins to deal with threats. Any taxes you manage to collect from them is just a bonus. Place it between waves so your peasants have time to finish it before a wave arrives.
Once these other tasks are done, if you find you have 5k or more, then you can upgrade your palace to level 3 to add another temple. I'm not sure whether Helia or Lunord is better since you're basically just defending your settlement; I chose Solarii under the assumption they would be a little tankier than Adepts. Alternatively, a better idea is to just build another Warrior's Guild and buy more paladins.
We're not going to worry about upgrading either our Dauros or Agrela temples, but we are going to make extensive use of their level 1 spells. Any time you see a paladin tanking more than one ratman champion (or even one champion if the paladin is low level), or a few basic ratmen, then cast Stoneskin on them. This will drastically lower the damage the paladin is taking, which lowers the pressure on their healing potion stock and your treasury. If a paladin dips to low hp (say, half), then heal them yourself. Spending 1000 gold on temple spells is a net gain for you if it saves a paladin's life, because that paladin already has experience. If you see a taxman being harassed, then consider the economics of casting stoneskin and/or heal on them. A 1k gold return is worth a 200 gold heal.
Pick an edge of the map close to your town that is NOT near a line of ratman advance, and build a trade post there. Don't spend gold to defend this post, just rebuild it when it is destroyed and the destroyers move on; the occasional swarm will head over specifically to wreck the one building (bonus: fewer rats swarming your heroes at one time). Even one tradecart will net more than the 500-ish gold to build a "first" trade post.
When a threateningly large swarm of ratmen gets near a Warrior Guild, use Call To Arms to pull all 4 resident paladins home. They'll get to work and the concentration of force will help them survive. Don't forget to stoneskin and heal them.
If a high level paladin (8-ish) is alone and surrounded, don't panic. Check whether they are doing okay and have some spare healing potions, and if they seem to be alright then just hit them with stoneskin and let them get experience. Keep an eye on them and use Call To Arms to pull them out if they are about to die.
If you have spare cash, then you can place a cluster of 2-3 Ballista Towers in a ratmen attack path. They can support existing defenses and impede any swarms they don't destroy outright.
Around Day 25, Rhodan and swarms appear all over the place. If you have two or three paladin guilds backed by a Lightning Storm or two, you should be fine. Rhodan himself isn't much of a threat; one of my level 8 paladins basically solo'd him with a little stoneskin and healing support from me. Once the swarms are under control, the new broken sewer mains can be torn down in short order.