It turns out that the land the Jarl's steward sold you has old stone walls surrounding the building site. These walls and and the five towers they support are now yours to use as shelter as you build your fancy new Hearthfire house and to use as a small fortress/village after the house is completed. You may even build a profitable mine near Windstad Manor or farm at Heljarchen Hall as you see that the nearby land is up-for-sale for these purposes.

The Windstad Mine Patch is for full compatibility with Windstad Mine SSE found here: http://www.nexusmods.com/skyrimspecialedition/mods/4160/?
The Heljarchen Farm Patch is for full compatibility with Heljarchen Farm SSE found here: http://www.nexusmods.com/skyrimspecialedition/mods/4108/?

There is a lot to describe here so I will Update the description in more detail soon. For now see the uploaded images and the short description below:

-Walkable defense walls and 5 usable towers

-Auto-Outfitting mannequins set player-designed guard uniforms (Anything put on a "Uniform" mannequin and/or in it's lower "Misc Gear" cabinet will be equipped to the corresponding guard group, equips on cell load)

-Magic portals to Winterhold College (Voidstone keys are located on top of the partition walls in the Archmage's quarters and unlocking the portals enables the magic using NPC guards,staff/merchants for that location)

-"Outdoor" armory/shops/tavern/living quarters including spare beds for followers and cozy storage-safe rooms for the player (5 spare beds at Heljarchen Hall and 9 spare beds at Windstad Manor including the player bed)

-New guard/staff/merchant NPCs (Day/Night 2 shifts of 4 fighter-guards and 1 mage-guard, Blacksmith, Enchanter, Apothecary, Tavern Keeper, Bard and General Merchant)

-NPC daily schedules and guard patrols (Eat/Sleep/Work/Freetime schedules, guards patrol walls, merchants work at shops and a tavern)

-Random dragon and bandit attacks (Opening any door in the walls has a small chance to trigger an attack on the fort, each trigger event has a random chance to spawn 1 dragon and up to 4 groups of 3 bandits all spawned at different locations, all enemy NPCs are leveled)