Analysis of the construction of tower houses has focused on two key driving forces. The first is that the construction of these castles appears to have been linked to periods of instability and insecurity in the areas concerned. In Scotland James IV's forfeiture of the Lordship of the Isles in 1494 led to an immediate burst of castle building across the region and, over the longer term, an increased degree of clan warfare, while the subsequent wars with England in the 1540s added to the level of insecurity over the rest of the century. Irish tower houses were built from the end of the 14th century onward as the countryside disintegrated into the unstable control of a large number of small lordships and Henry VI promoted their construction with financial rewards in a bid to improve security. English tower houses were built along the frontier with Scotland in a dangerous and insecure period. Secondly, and paradoxically, appears to have been the periods of relative prosperity. Contemporary historian William Camden observed of the northern English and the Scots, "there is not a man amongst them of a better sort that hath not his little tower or pile", and many tower houses seem to have been built as much as status symbols as defensive structures. Along the English-Scottish borders the construction pattern follows the relative prosperity of the different side: the English lords built tower houses primarily in the early 15th century, when northern England was particularly prosperous, while their Scottish equivalents built them in late 15th and early 16th centuries, boom periods in the economy of Scotland. In Ireland the growth of tower houses during the 15th century mirrors the rise of cattle herding and the resulting wealth that this brought to many of the lesser lords in Ireland.