From The Heroes of Might and Magic III wiki

Class	Earth Magic
Basic cost	12
Duration	2 Rounds
Level	3rd

Creates an impassable wall, that can only be removed by expert Remove Obstacle.

Level	Effect
None	Wall is 2 hexes high
Basic	Cost reduced by 3
Advanced	Wall is 3 hexes high
Expert	Same as advanced)

[edit] Comments

Underrated spell.
It has two main uses: to block enemy walkers from going down a narrow path, and to protect shooters from attacks.

The latter capability can be VERY useful against the AI in certain situations if you have a large stack of shooters (Titans being the most obvious). If you place them in the upper left corner, you can cover the Ammo Cart as well with the Force Field, and if you are facing 2-hex non-ranged enemies who are slower than you (or can be made slower by Haste, Slow, and/or Prayer), you cannot lose even against a much superior force. A human opponent of course would be able to circumvent your plan in a number of ways...

Note: Spell duration is NOT increased by any of the spell-duration boosting artifacts (Ring, Collar& Cape of Conjuring).