By mutual agreement, or at the end of any day following the playing of at least 35 turns of daylight (turns lost because of late start dice are not counted), or by decree of the umpire, a field battle can be converted to a siege. This can only happen if at least 30" of Fieldworks or Fort are present in the box (yes this means you can't besiege Fortress Monroe), failing this the strategic attacker must withdraw.

Where the units are at the point then battle becomes a siege - which is effectively when the attacker ceases attacking and the defender elects not to sally. Only the trace of the front lines need be noted. This box is now partially held by both sides, and the campaign umpire will decree (on the basis of where the units are) who holds which supply lines out of the box. This may mean both sides now own friendly rail lines into the box, thus facilitating supply, and the introduction of reinforcements, including a siege train). Both sides are automatically provided with hasty works to the frontage of all the units present, less any works already held by that side (and facing the right way).

In subsequent player turns either side can expend marches in the box as normal to entrench, or can expend one march to initiate another field battle. Units moving into the box by friendly-held transport line are added to the forces available at the start of this battle. They don't initiate the battle merely by arriving if the come along a friendly tranport link, that needs another march. Units coming by enemy-held routes dice as normal if coming in with a march, or arrive at 1pm if supporting. Both sides can set up their initial forces anywhere behind the trace of their respective works. Battle then plays out as normal for another 35 hours, etc.

Since both sides in a siege will be heavily dug-in, they will each get an intelligence report one level worse than they are theoretically entitled to - so a complete report is the best you can get. Fragmentary remains the worst.