To restrict trespassing, the height of the fence is critical. The minimum suggested height is 6ft and it can go as high as 12ft. For vacant lands where agriculture is to be practiced soon, it is recommended to go for a 6ft high fence using either a stone/concrete or iron posts. If budget permits, concertina coil can be installed over the top of the fence to reduce trespassing occurrences significantly.
Another middle solution which is cheaper than installing concertina would be to install razor wires on an incline fence post on top of the main fence-able area to prevent trespassing.