The Norman invasion was the greatest catastophe in the history of England (and of the British Isles); because the Normans (for all their higher intelligence and talents e.g. as warriors, administrators, architects) were basically evil. As a ruling class, and in stark contrast to the Anglo Saxons and Celts; the Normans and their legacy have been unspiritual in nature, hostile to real Christianity.
Even nowadays, it can be seen (in the eyes) and felt (by the heart) that the Norman descendents and those assimilated to them have 'something missing', are lacking in 'soul'. I would regard this as related to their service to evil. Face-to-face, they are not fully human; although often good at pretending.
Yet, starting from this insight, we can ask why this was 'allowed to happen'. Terry Boardman argues throughought his work that - yes, the Normans were evil, and also they set-up an ultimately beneficial interaction.
This mortal world is not meant to be a paradise of ease and comfort and pleasure - but is designed as a place of learning from experience; aimed at attaining and benefitting our eternal resurrected life.
Thus evil is not just tolerated, but may be deployed for optimal benefit (within necessary constraints of human agency) - and something of the kind happened with the Normans. (Note: The 'process' actually occurs primarily at an individual level, not by groups; and every individual is a mix of good and evil - even though they will serve one side or the other, overeall. Clearly some individual Normans have repented, been on the side of Good, and spiritually beneficial.)
The Norman ruling class provided skills and perspective; a challenge which the English spirit could grow-against; and that led to England becoming a world power and achieving the world-transforming industrial revolution - and at the spiritual level bringing-forth the 'consciousness soul', with its separation of subjectivity from objectivity: this providing the ultimate basis of spiritual freedom (a necessary step towards greater divinity).
But for the English to go beyond the consciousness soul and on towards the destined Final Participation required that the English spirit overcome the Norman - and this has not (or not yet) happened. The Norman-derived English Establishment have instead (pretty much) ideologically taken-over the world (US, UN, EU), and are well-advanced in imposing their long term goal of totalitarian materialism in service of their evil masters.
So, the Normans have done a great deal for us in a spiritual sense, but that good now lies more than two centuries in the past; and it is by now long overdue that the English spirit resisted their long-term idological colonisation; and overcame their corrupt-and-corrupting rulers.
What we need to do is the same whether or not you agree-with or understand the above analysis; but it is helpful to know your real enemy, esepcially when they are so powerful and influential - and therefore avoid mistaking your enemies for your allies.