To build in swamp your magi have not drained before, they might steal ideas from the architects of Venice. Your magus' good Craft: Architecture might let him know their methods. Here is a summary of them. Better, more detailed descriptions you can find e.g. in good travel guides like the TCI Guida Rossa of Venice from 2007.
The Faxekalk found in churches on Zealand is not as dense and impermeable as the Istrian Limestone used by the Venetians, though.
The idea is to get to the bedrock with PeTe, keep the hole dry enough to build a foundation with Stone to Clay (and repeatedly wait for the sun to set). That should get me to a jointless base and jointless walls and ceilings that would keep away the moisture and water.
You could do a lot of that underwater? Shaping clay in a water filled hole seems pretty reasonable. If you can manage to be able to breathe under water.
With a glass specialty displacing a huge amount of mud with a giant glass block.
(3base + 1 touch +2 sun+ 1 size)
Makes you a glass block of 100 cubic paces
(2 +1touch +1 Conc +1 Size +1 Glass)
Drives it into the ground with some force.
Make a spire, Pile drive it into the ground until you see stone, Dig through it.
(3+1glass+1 touch) = 5 Pe Te to dig through it pretty quickly
THen do your thing with something like 100 tons of pressure pushing down to keep the water out. Or for the last pit turn the glass to clay to make a seal.
All within your focus so you should be able to pull a lot of it off.
Note that for draining using craft magic, as opposed to retaining the drained state, none of the actual structures need to be built, because the magic elides all the interim steps. On the downside, since draining takes a long time, the spell level goes up.
So craft magic that can drain an entire marsh will just do it. And if local conditions allow that land to remain drained, the job is done.
(Although PeAq can also do this. And it might be easier to deal with drainage by recasting PeAq every now and then.)
using CrTe, first create a ring, hollow on the inside, with duration of moon. Then use MuTe with duration day to turn a nice, non-porous rock into a liquid, and pour it into the mold you have created, displacing teh water which will spill over the outside of your ring. when the moon period is up you should have a nice foundation in place.
Alternately use lime, volcanic ash, and salt to form concrete using the water as part of the formation process... Roman concrete.
Minor magical focus on glass? Glass can hold water very well, and magical glass should be able to hold back water as effectively as the glass in an aquarium.
MuTe base 3 "change dirt to stone", +1 touch, +3 duration moon, +1 target part (you're targeting a patch of earth) for total 20. Turns up to 10 cubic paces of earth into 1/10 the volume of magic glass for moon duration. With Mu (5x2 for focus) and Te 12, you should be able to research "dam of glass" and line the sides of any pit with glass.
If you're on Zealand, it might be worth looking up historical castles such as the wiki list and see if there's any clues to materials from what they built. OK, most of Denmark's historical castles are 15th century onwards, but you might get some hints.
Ah - Vordingborg on the south coast of Zealand was originally wood in the 12th century, and Valdemar II (the current king in 1220) rebuilt it in brick. Therefore, turning clay into brick (which I believe ReTe can do quite effectively) is what is proven to work on this island during the Ars 5th time period.
Fired brick is rare stuff in medieval architecture, which relies on bricks dried in the air instead. But your SG might determine (perhaps after a good stress roll), that your magus knows that technique because of his high Craft: Architecture.
If he does, using ReTe Craft magic to turn clay into fired bricks is an excellent approach. Its results are permanent without vis, and may be impermeable and very solid.
I read a book on Danish history recently which said that brick kilns arrived in Denmark in the 12th century. It makes sense that northern European countries where air drying bricks would take a long time would adopt this, and certainly surviving medieval Danish architecture uses a lot of red brick - Thank you Archmage wiki!
Also, Rego Craft magic doesn't require you to be that skilled in the craft you're replicating, just good at Finesse. The sidebar on Covenants p49 says "Rego magic can make any change a mundane craftsman can effect, even when the magus is limited by lack of tools, time or skill". Therefore you don't need heat or a supply of dry air, brickmaking tools or any particular skill in brickmaking - just cast the spell to rearrange the matter and get a good Finesse roll!
"The phantom brickmaker" - ReTe 2 "Shape and form dirt, as if a craftsman had worked it", let's add the magnitude for stone as we're turning clay into a form that's as hard as stone, +1 touch, +2 group - total ReTe 10 turns a small pile of clay into ten bricks. You will exhaust yourself trying to spontaneously cast this, but if you have the stamina and a favourable aura maybe you could cast the individual version as a non-fatiguing level 4 spont and make your bricks one at a time?
Found the quote, it is in the project redcap's House Rule page...
The real condition in a target leaving the circle or it is physically broken. You are not targeting the workers or the material they are removing (detestably on the last part, I don't like to allow that much granularity in the targeting of circles).
There's a lot of good ideas here; I just want to poke in and add that a lot of my new players have a tendency to forget ceremonial magic, which can give you a nice little bonus depending on your Artes and Philosphy... that casual little +2 or +4 or whatever (depending on your abilities, though looking back a lot of people only have AL 1 and Philo 0 in my game) can be very effective at succeeding the trickier bits. Also, having a pre-defined stone/clay circle in the swamp will make it much easier to keep recasting circle spells.