"Head to Windmill" is an objective in the quest The Bloody Harvest in T.K. Baha's Bloody Harvest DLC
Head on through the now open gate. On the other side you will find a pair of Vending Machines to heal up and restock at (if needed).
As you head down the slope you will find yourself beset by a few Pumplings. Deal with them and then go over the drop. This will take you into The Pumpkin Patch. Go into there and Zombie TK Baha will contact you. He confirms that the Pumpkin Kingpin lives here but assumes he must be asleep underground. He will tell you to head for the Windmill so you can flood the field and force him up for air.
As you start across the field, more Pumplings will appear. Kill them and go across the field to the West. Go on up the slope and you will find the next part you need to work on.