This is what I really want to do: have "monster's" created in an underground lab. DNA printing/splicing, etc. Science gone wrong. So I need hi-tech for that. At the same time. I want it to be set in a lo-tech/medieval type world. How can I do this and have it make "sense"?
ETs experimenting with the planet's life? A scientists that makes a GMO but in some crude way like on the Island of Dr Moreau?
This really depends on whether you're writing sci-fi or not. If you're okay with the idea of wring something that goes a little more toward the fantasy realm, you can explain all sorts of weird things using alchemy and pseudo-magical processes (heck in Lord of the Rings, Saruman bred and incubated and entire army worth of ugliness at Eisengard). If you don't want magic at all, maybe look at what real-life alchemists tried to do and what methods they used - then posit those methods actually stumbling on something that works and then blows up in their faces.
I suggest you read Kage Baker's In the Garden of Iden. The situation is exactly as you describe—and what a story! Of course don't copy her. Just enjoy the book. I'd be very surprised if you didn't.
Oh my word! I bought the e version right away after reading two pages, I was hooked! I love the voice. I love the way she just says this is how it is and that is how it is then. I thought that may come off as not believable, but it does. Wow, what a great find, thanks.
My first thought is what can I steal? I better lay off the book, until I know what I want to do first. Otherwise I know I'll feel discouraged and tell myself why bother trying. Not good for me. I'm wondering if I take mine more fantasy and maybe there is a certain herb/metal in the ground that has found to give birth to all sorts of "nasties"?
Far easier to create a monster through abuse than by nature. Though you could always go with the route that some apex predators was brought by foreigners, escaped and carried on a terrible campaign of bloodshed and hunting as a growing pack. Wolves are smart and that would be my start.
I'd have castles of steel instead of stone, wizards who wield science instead of magic and cybernetic knights in armour. Your DNA tinkering could be done by a wizard.
It wouldn't make sense at all. I don't mean to be a pessimist, but this seems like something readers would shoot full of holes within seconds. My recommendation? Look up what was considered the "advanced" sciences in the medieval era, like alchemy or sorcery. Build around that.
One way of looking out it, may be modern Earth being destroyed and human civilization puts itself back together as you has said over a period of decades. However, in the underground bunkers the governments are still operating, watching the rest of humanity and increasing their scientific capabilities which may include creating these monsters but then it all goes wrong and they get loose.
It could seem more plausible by dropping hints that the technology exists before unveiling it. Although I also really like the idea of having wizards or alchemists stumble across some highly advanced, modern era science. They could discuss real world genetics, mingled with some fantasy-based ideas, as they conduct their research.
Two things as my two cents : 1) you don't need high-tech or magic to create monsters if there are already a bunch of dangerous animals in your world. Decades or centuries of targeted breeding and cross-breeding will make real monsters even from the most harmless species. To make it realistic you need to give your evil monster-masters enough time and resources to experiment a lot. Most of the cross-breeds will produce weak or nonviable descendants and most likely those that survive will be infertile. Only a very small number of them will be a good base to work with and be the parents of the next generation. 2) there could be a very big difference between the technological levels of different geographical locations. We all know from our own history what happened when a developed nation expanded to new lands where the natives were on a much lower tech level. Hope these help.
