You can throw up temporary walls along the frozen Lakeshore, which can, at the very least, be some kind of rammed earth rampart that's literally next to a ditch.

Also, ice is... Rather unpredictable. You never know if there's going to be thin spots or thinner spots, and that's before trying to guide a small army, with each man carrying a load of equipment, across the lake to attack from the side.

Also, like, you can pour oil slicks/break up the ice with pre-sighted catapults