What is the most succinct formula to concatenate D1:D64, which contain one
character each?

I thought the following array formula would do the trick:

=concatenate(index(D1:D64,row(1:64)))

It didn't work. Well, I guess I understand why. CONCATENATE({"a","b"})
doesn't work either.

I finally resorted to a macro. But I'm still interested in an Excel
formula.

(Excel 2003 SP3.)