Numerals
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ŪNU > (Castilian) uno; 'un' is down to apocopy in proclisis. However, in certain circumstances UNA also suffers apocopy - e.g. un alma (el alma), un hacha (el hacha). This is probably a grammarians' concoction (see the sections on Articles for an explanation of the use of 'el' as a feminine article).
Alguna and ninguna also apocopate in the same circumstances. We have este alma and ese hacha, but aquella does not suffer apocopy.
DUOS, DUAS > dos, duas/dues (Old Castilian) > dos. In Portuguese, Catalan, Roumanian and Provençal both feminine and masculine forms are preserved. This is also the case in several Spanish dialects, and the Italian due < DUAE (Latin feminine plural).
The numbers 3 - 15 follow simple phonological development in Castilian. The only apparent morphological change occurs in the word for 5:
QUINQUE (C.L.) > CINQUE (V.L, through dissimilation - see Italian and French). The final -co is something of a mystery, but it may be down to assimilation with the preceding cuatro.
The number 16 took the following route:
SEDECIM > se(d)ze (in Old Castilian); dieciseis came about to avoid confusion with the Old Castilian verb to seize. Parallelly,
SEDECIM > seize (French)
setze (Catalan)
sedici (Italian).