You must understand that it wasn't until the Romans took over the Gallic and Germanic areas that the lands became proto-urban.

Until then, they were very much village based and agriculture driven. I guess Alesia is probably the most famous in Gaul. However this wasn't really a "city" in a Roman or Greek sense.

The Gallic people's certainly were becoming more urban, but the Roman occupation of these lands certainly sped up the process.

This is sort of a complicated question depending on where you are talking about, because the nature and extent of pre-Roman urbanization in many parts of Europe is hotly contested. I'll go from most to least straightforward:

Well before the Romans conquest the "Mediterranean band" of western Europe had been well urbanized. The old standard explanation of these considered them to be more or less entirely colonial entities, owing their development and character to foreign influence. And indeed many cities, such as the Phoenician Agadir/Gades/Cadiz and the Greek Masalia/Marselha/Marseilles, do have substantial colonial aspects to their foundation. However, more recent research tends to stress the indigenous contribution, either in terms of the non-colonial cities such as Tartessos or in the essentially dialogic nature of the colonial cities. The foundation myth of Marseilles, for example, has it created due to a pact of friendship between a local Ligurgian king and Phocaen merchants and refugees. Archaeologically, setting up a dichotomous relationship between foreign and native material is generally poor methodology, but generally speaking colonial cities had different material matrices than those back in the homeland.

The next least ambiguous case is Britain, where in southeast England the oppida had developed into what I will call political rather than demographic urbanization. What I mean by this is that there were centers of concentrated political power that represented foci for group identity, and even a suggestion of "palaces" in the period between Caesar and Claudius' campaigns, but this did not represent a major concentration of population.

The big question mark comes with the bit in the middle, as it is quite heavily debated to what extent places such as Autun, Leon, and Orleans actually constituted cities. Some stress that these were essentially just scaled up versions of what we see in Britain, while others argue that they were true cities. It is unfortunately not a debate I am competent to resolve, but believe the pendulum has swung towards accepting them as real cities.

Now of course this is purely western Europe. The urbanization patterns in southeast Europe are extremely complicated. Plovdiv, to take one example, has been an urban area since the Neolithic, and generally speaking the more research there is on the Dacians the more complex are their political and social structures.

A good general source for a (very) broad history of Europe is Barry Cunliffe's Europe Between the Oceans.

Question for the mods: why is this thread tagged as NSFW? Doesn't seem like it needs to be.