What kind of wall do I build on marshy ground?

[BCS]

I don't know how to fortify on this kind of terrain.

[RubyCante]

you need to build bridges on it,then you can build normally..
if the bridges collapse,you lost everything on it lol

[Defektiv]

You can use a moisture pump to turn it into regular soil you can build on without a bridge too. They are fragile and take a good amount of power though and take a long time to work. If it's only one tile that you need to clear marsh from you can put the moisture pump directly on that tile or put a bridge down on it then the moisture pump on top. It clears the first tile as soon as you build the pump on the tile it sits. More than a few tiles you need cleared and you'd have to wait a lot longer for it to expand.

[liosalpha]

or install a terra forming mod. letting you clearou and place water to your hearts content.