Slitherine

Is it possible to add units? The heavy tank unit in the unit_eng.txt doesn't seem to do anything/show up. There's also SpareInfantry, SpareArmy, etc. Can these be used?

I am afraid you then have to replace one of the existing unit types. The sequel Commander - Napoleon at War has generic support for more than the unit types in game though.

I'd like replace the Battleship with Artillery, but I don't think I'll be able to place it on land as it's a naval unit type. Is this correct?