Ambarkanta could not allow of it: as I said in my commentary (IV.253):
It is not indeed explained in the Ambarkanta how the Valar entered the world at its beginning, passing through the impassable Walls, MORGOTH`S RING - AINULINDALË - Version C - 27
and perhaps we should not expect it to be. But the central idea at this time is clear: from the Beginning to the Great Battle in which Melko was overthrown, the world with all its inhabitants was inescapably bounded; but at the very end, in order to extrude Melko into the Void, the Valar were able to pierce the Walls by a Door.
The far more complex account in the new work of the movements of Melkor and of his strife with the Valar is an indication at once, therefore, that shifts have taken place in the cosmology.
In the Ainulindalë proper it is now told that Melkor entered the World with the other Ainur at the beginning - he 'was there from the first', and claimed Earth for his own (§23); but he was alone, and unable to resist the Valar, and he 'withdrew to other regions' (§24). There followed the labours of the Valar 'in the ordering of the Earth, and the curbing of its tumults', and Melkor saw from afar that 'Earth was become as a garden for them'; then in envy and malice he 'descended upon Earth' to begin 'the first battle of the Valar and Melkor for the dominion of Arda' (§§26-7).
The words 'Earth was become as a garden for them' are not to be interpreted as a reference to the 'Spring of Arda', for the description of this follows in the Words of Pengoloð; where appears also the wholly new element that Tulkas was not one of the Ainur who entered the World at the beginning, but came only when 'in the far heaven' he heard of the war 'in the Little World' (§31).
Then follows the building of the Lamps and the Spring of Arda; for Melkor had fled from the Earth a second time, routed by Tulkas, and 'brooded in the outer darkness'. At the end of 'a long age' he came back in secret to the far North of Middle-earth, whence his evil power spread, and whence he came against the Valar in renewed war, and cast down the Lamps (§32). Then the Valar departed from the island of Almar in the great lake and made their dwelling in the uttermost West; and from Valinor they came against Melkor again. But they could not defeat him; and at that time he built Utumno. There are thus four distinct periods of strife between Melkor and the Valar, and he departed out of Arda and returned to it twice.
We are brought therefore to the forbidding problem of the underlying conception of the World in this phase of my father's later work. In the original Music of the Ainur in The Book of Lost Tales Ilúvatar 'fashioned [for the Ainur]
dwellings in the void, and dwelt among them' (I.52); at the end of the Music he
'went forth from his dwellings, past those fair regions he had fashioned for the Ainur, out into the dark places' (I.55), and 'when they reached the midmost void they beheld a sight of surpassing beauty and wonder where before had been emptiness': 'the Ainur marvelled to see how the world was globed amid the void and yet separated from it' (I.55-6). This may not be a simple conception, but it is pictorially simple. In Ainulindalë B it was MORGOTH`S RING - AINULINDALË - Version C - 28
not changed (V.159). In the Ambarkanta 'the World' ( Ilu ) is 'globed' within the invisible, impassable Walls of the World ( Ilurambar ), and 'the World is set amid Kúma, the Void, the Night without form or time' (IV.235-7). I take these accounts to be in agreement. 'The World' comprises 'the Earth' ( Ambar ), the region of the heavenly bodies that pass over it, and the Outer Sea ( Vaiya ), 'more like to sea below the Earth and more like to air above the Earth', which enfolds or 'englobes' all (IV.236).
In C, likewise, Ilúvatar 'went forth from the fair regions that he had made for the Ainur', and they came into the Void (§§10-11). There Ilúvatar showed them a Vision, 'and they saw a new World ...