Wall

You build a defensible position out of the land itself.

While making contact with the ground, and inside your transmutation circle you can create a 5 foot cube of a solid element (wood, earth, or ice). If a creature is currently in the target location, friend or foe, they decide to stay put and move with the wall as it forms, or let themselves be shunted to an adjacent location of their choice. This block has 25 hit points, and an AC of 20. It lasts until it is destroyed or recast, upon which it crumbles into its base components. 

The number of walls increases by one for every spell slot above 2nd. Medium to large characters can use these walls to achieve half cover, while small and tiny characters gain 3/4 cover.