The story
May either be a direct compound of N (the initial letter of nigger, or an ellipsis of n-word) + tower, or a blend of n-word + tower. Alternatively still, possibly a syncope of N-word tower. Unlike n-word, this term may resist being spelled with a lowercase N to a greater degree, since the format of such towers often explicitly features image macros of capital Ns.
Etymology from Wiktionary, CC BY-SA 4.0.