I know - but nobody can understand cubes, therefore, the way I look at it is, that cubes is likely to be wrong and if the only reason that cubes was suggested was because the letters fit, what happens if there is an alternative answer for one of the crossers - and here, if the E is wrong.
Imcidentally 99 in Roman numerals is not IC, it is XCIX, you have to write 90 (XC) first and then 9 (IX). You can't just take 1 from 100. Similarly that is why 450 = CDL rather than LD.