Dreichholm Brigade to the rescue!

'How dare you!'
Dietrich von Spülge entered the corridor of the High Chancellery shouting all the way to von Südflosse's office, and when he finally arrived into the room, he showed no intention to stop. 'How dare you not warning me that my hometown has been fallen to Flossian hands, you ruffian, you, you... Son of a baker, darn clerk, you will pay for this!'
'Excuse me, Herr Generalleutnant, I am writing orders', Erik von Südflosse told him. Spülge's rage was gone in a second. 'What sort of orders?', he asked, hoping for the best.
Von Südflosse signed the papers and then sealed the envelope. 'Orders for the Dreichholm Brigade to march west and liberate Grübsdorf from the enemy siege. By chance, take back Spülge too. Look, I knew I couldn't stop you from attacking those wretched Flossians, let us make it legal once and for all. In the meantime, I'm sending orders to dear old Hohenspitz too: he will attack as soon as he can. This is a level of threat we did not expect: don't turn it into peril.'
'Sir', von Spülge bowed then left in a quick pace. He rode to Dreichholm immediately and started marching towards Grübsheim.

The fort of Grübsheim was held by 6 battalions of foot - a fusilier and a militia regiment -, plus a squadron of dragoons. They were facing the full force of the Union, but were keen to hold the walls. They knew help would come, but did not know when. Judging by the fate of Spülge, their only option was to hold.
The Flossian army - at least the foot as the cavalry brigade decided to spend a few days in Spülge, in the inhabitants' empty homes - slowly surrounded Grübsheim and made itself ready to storm the walls.