While we can have a fair idea what transalpine Iron Age buildings or agglomerations would have looked like, we are immediately confronted to two problems : the first being the choice of materials, the second the lack of continued use or maintenance.

When it comes to ancient Celts (Hallstattian and LaTenian architecture in particular), the materials of choices were wood, hay or thatch, cob, dirt or rammed earth. These were abundant, immediately available and adapted to the environmental needs but are fragile and certainly not well suited to stand the test of time.

Stone wasn't really used for masonry or big structures, but rather for foundations, small walls or fortifications (namely with the murus gallicus, an architectural technique that wasn't used everywhere in the Latenian horizon and used stone in addition to wood and dirt)

Even Mediterranean agglomerations in Gaul, that used stone more importantly for big structures and monumental fortifications (either because the material was more adapted due to lack of wood and/or as an imitation to Hellenistic features) still used perishable materials for the bulk of the constructions.

It means that what could see of remaining pre-Roman architecture would rather look underwhelming (e.g. Ensérune's cisterns; Ambrussum's fortifications; Pech-Maho; Corent being understood that these are the state after archaeological campaigns and cleaning-up, having removed lot of dirt, grass, wood, rubble, etc. and put some parts back in place (hopefully).

Everything that made the bulk or even the decorations of the buildings (coatings, slabbing, facings, pavings, paintings, engravings, etc.) is found at best in an extremely fragmentary state or quite often entirely missing (hence why sizes of buildings or structures are often deducted from posts holes placing), so it takes a lot of archaeological and historical work and guesswork to go from this current state of Entremont to [this reconstruction] (credit : Jean-Claudes Golvin) or from this archaeological campaign at Paule to this reconstruction

Now the choice of materials isn't the only reason for the lack of remaining monumental features : when we think of ancient monuments, we might think of them as having stood the test of time for being exceptionally built, directly transmitted from previous eras.

But that's forgetting that monuments have their own history : a Roman amphitheatre or temple could be repurposed as fortress, graveyard, church, habitations quarters, more or less heavily transformed and repurposed allowing them a continuous existence and maintenance even while in a state of disrepair, pillaged, etc. along a fair deal of survivor and selection bias, monuments entirely or almost entirely disappearing,

Most of this story can be invisibilized as the taste for classical humanities required more and more, from the XVIIIth century onwards, that these places be freed from their medieval and modern modifications as far as it could be done and sometimes partly reconstructed (the Arenes of Nimes were thus cleaned out of medieval fortifications and habitations and rehabilitated as a classical monument). For some monuments, this reconstruction is even more critical to our current perception of them, such as Hadrian's Wall in spite of its "organic" story and how its current state is only part of it.

After the conquest, Roman culture utterly triumphed : agglomerations were either abandoned for newly built cities or wholly transformed as such, cultural references and understanding of monumentality changed in a way that maintaining old features that didn't fit modern tastes would have been absurd, leaving much to disrepair. And even while there were cases of reoccupation (notably during the Early and High Middle Ages), at this point centuries of abandonment (the choice of materials meaning that quickly led to destruction by neglect) or use as quarries made them essentially disconnected and there was really nothing, either in memory, prestige or actual building to maintain or preserve.