Salem

Can we build something on top of a mysterious lillypad, something like walls?

Yes you can. But Lillypad has to be conected to the land so im pretty sure you cant just place it randomly in the middle of a lake for example.

Great, thanks for the answer

You can also place it somewhere in the middle, but if you want to build a wall over a river every tile needs a lillypad to connect one side of the river with the other.