Where are the enclaves in Europe where American expats tend to migrate.
We certainly don't want to retire to an "Americanized" community, but would still like to have a few neighbors from the homeland.

Americans are everywhere. I would recommend that you not use that as a prerequisite in choosing a locale. Instead, draw up a list of places that intrigue you and go visit (for a couple of months). Then, make your decision.

I think that it is nice to have an expat community to meet with, someone that shares your culture. If there is none where you live, it makes it harder when things go wrong, and they will. However, unless it is your goal, watch out for places where you can isolate yourself totally from the locals.

Do you want enclaves of retired US citizens? That will be a lilttle harder to come by, as Europe is more expensive than the US, and Americans tend to head to countries that are less expensive.

In Mar del Plata there is no ex-pat community, although there are foreign people living here, everybody just integrates into the local culture. I think it makes it easier to adapt then to try and continue like in your own country. I find that the ex-pats in BA only get together so that they can complain about everything and anything.

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A wonderful place to live

By community, I assumed the OP meant a social community, not a physically separate one. You know, some people to have coffee with, ask questions of, talk about back home a bit.

While most European cities have substantial expat communities, one of the best I've found is in Prague, Czech Republic, which has the very real benefit of being relatively inexpensive, although suffering every day from the rapidly declining US Dollar. There is a fairly large American population there, out of which one can usually find coffee- or bar-buddies, significant aid in dealing with the massive Czech bureaucracy, etc., and help in the always difficult transition from one culture to another.