To keep a field as safe as a castle keep, first lay out a double line of hedges or thorny bushes along the perimeter—think of them as a living battlement that slows any would‑be invader. Dig a shallow trench, about a foot deep, behind the hedge; it’s a simple but effective obstacle for a fast rider or foot soldier. Place a few watch posts at the corners or at intervals—short, sturdy wooden towers with a clear view of the surrounding land. From each tower, a rope or chain of arrows could be lowered to deter crossing. If you have a water source, position a moat or dry ditch in front of the most vulnerable side; the gap can be bridged only by a controllable drawbridge. Finally, keep the crops planted in a way that creates a natural choke‑point: rows that force any approach to narrow down to a single lane, giving defenders a clear line of sight. This mix of natural and man‑made defenses is as practical today as it was on a medieval field.

I agree, the most prudent thing is to scout the land first. Draw a simple map, note the natural ridges and any watercourses—those are the natural lines of defense. For the hedges, instead of a continuous wall, consider a ring of taller shrubs at the most exposed points; they’re cheaper to grow and still present a barrier. Trenches only where the enemy would have to cross a clear path. Watch posts must be real posts—just a sturdy timber frame with a small platform. Have a volunteer or two keep them manned at dawn and dusk; a simple bell or flag at the top will signal when someone is approaching. The rope‑dropped arrow idea is clever, but yes, trial on a small section first. Even a short line of arrows can deter a few men, and you’ll learn if the rope slides or if the arrows fly too far. For the crop rows, stagger them at about a meter apart; that way, if one line fails, the next still holds the field together. Keep everything tidy, check the plan regularly, and you’ll have a defence that’s both efficient and manageable.