Ice

From GenieWiki

Ice is a terrain introduced in Age of Empires II: The Conquerors. It may be crossed by land units, but it cannot be built on and is impassable to naval units. The new terrain was introduced alongside a range of other frozen snow terrains, allowing for a range of new winter maps and scenario design possibilities. The terrain can be used in certain tricks, such as breaking through ice in Ingo van Thiel's Gyda's Challenge. It also appears as a beta terrain in the original Age of Kings which can be unlocked through the use of map copy.