I play this game with the entrenchment markers of Avalon Hill's Guns of August. I also use the Russian artillery. and even add a couple of extra Russian artillery pieces. The idea is that it was possible the war could have bogged down into a WWI style slog, if the high commands were less offensive obsessed. Also, fortification was a significant factor in the war, at Leningrad, Sevastopol, Kursk and many other places.
Entrenchment level 1 protects one corps, EL-2 protects two corps or a Russian army, EL3 protects 3 corps or two Russian armies or a Russian army and a corps.
Each movement impulse, entrenchments can be advanced one level. Thus the E-level can advance by +2 in a two-month turn.
Full Entrenchment shifts the column odds one left.
River, Rough and Major City defenses shift the column odds one left.
A Fort is created by three fully entrenched corps which remain stationary for a movement impulse. Forts shift column odds two left. Once completed, any number of corps receive their protection.
Entrenchment/Fortification is not permitted in Snow or Mud months.
Mud also shifts column odds one left.