The need for siege engines directly comes from the need to overcome fortified obstacles, espcially walls or towers, that "passively" besieging can't deal with quickly or efficiently enough, for instance if the fortified party have enough supplies to wait for reinforcements or besiegers' weariness.While walled cities and strongholds were common in eastern Mediterranean regions and beyond by the Bronze Age leading to a spectacular development of poliorcetics, the branch of tactics on taking cities by force, that Roman warfare was largely in debt from rather than innovators, nothing really comparable existed in contemporary Europe.

By the late Bronze Age (ca. 1400), fortifications known as hill forts began to appear there : as their name, these were natural formations (hills, promontories, etc.) ehanced by basic fortifications such as eathern works, a stone line, palisades, brambles, etc. ( being worth remembering these walls probably served other functions of delimitation or even as monumentalisation of power) that is nothing preventing a resoulte assault to take place, but certainly obstructing it and at least making a potential attacker thinking twice about doing so. As we're talking of relatively small warring bands or troops, it's understable as siege warfare in a distant and dissimilar way to LBA Eastern Med, and altough these fortifications could be fairly sophisticated and developed especially in the transition to the Early Iron Age it was nothing warranting the use of siege engines if the social context would have allowed it (namely with what it implies in the existence of client craftsmen)

Walled fortifications in the more usual definition were associated with the development of proto-urban agglomerations in the Early Iron Age and the Early/Late transition in the western Mediterranean basin at large, but as well in Atlantic and Alpine Europe in what are generally considered the Celtic "cores" (altough the Castro culture of Atlantic Spain and the hillforts of Britain and Ireland are worth consideration, I'll focus on Gaulish warfare.

Stefan Fichtl propose to see late LaTenian walls fulfilling three distinct functions we could tentatively ascribe up to a point to Later Iron Age non-classical Western Europe as a whole : military, ostentatory and symbolical.

Military aspects are generally self-evident, but not as crushingly obvious a comparison with Archaic Greece or Italy would make it : the idea still seems most of the time to provide an obstacle to assult from footmen or horsemen or, by its monumentality, to dissuade potential attackers to even try not to face long sieges and thus not providing opportunities for an indigenous siege engineering. Marking physically the territory, especially by their building techniques (sometimes even fragilizing the walls for esthetic purposes or with sloppy craftmanship in remote part of the walls) or the sheer size of their walls or towers might have seen of prime importance possibly before a full military role, displaying leadership power and/or a communal capacity of social and resources mobilisation; but also symbolically marking a public space of power, religion and action for the community comparable to the Roman pomerium and generally to a sense of enclosure (much more attested in La Tene sacred or profane space, tough)

The de-urbanization of Alpine Europe between the IVth and IIIrd centuries was not paralleled in Mediterranean Spain or Gaul but without evidence for siege warfare in the strictest sense either : at most could it be said that by the late IIIrd century, Italian or Hellenistic influence did involve more classical walling as part of a whole "package".It is not to say that Gaulsl were ignorant of what siege warfare was : either by raids/migrations or mercenariship in all the classical Mediterranean basin from Carthage to Greece or Egypt, they were certainly familiar or at least spectators of poliorcetics. But as "specialist" mercenaries they were probably tasked with other military functions as units, or as attackers seems to have followed fairly passive tactics waiting for sorties to exploit of a starved ennemy to surrender. Coming back home, there was at least under the IInd century BCE little reason to carry what they could have learnd in classical armies to whole different strategic, but also ideologic, world : indeed, the sacrality of warfare in ancient Gaul (and probably the whole LaTenian world) favored open battle and warring prowess, things that wouldn't have been expressed with siege engines especially as they were associated with social groups outside the dominating warrior-aristocrats. Eventually, the colder and wetter climate of transalpine Europe might not have benefitted the torsion artillery used in the Mediterranean basin. The fact remains that there's an utter lack of archeological or historical evidence for siege warfare north of Alps, and thus, no need for siege engineery or specialist even as craftmanship develloped in these regions into incredibly sophisticated displays.

Still, by the Ist century BCE and the Gallic Wars, things had changed a bit. Caesar's account inform us on a variety of siege and counter-siege techniques practiced by Gauls : sapes and counter-sapes to either fragilize the walls (for instance, at Bibrax) or besieging works (the Roman ramp in Avaricum); use of stones or traits to remove defender from wall walks (archeologically evidenced at Bibracte), covering the turrets with laeathers and taking siege hooks away from Roman with laces (yet again in Avaricum) : while unimpressive compared to Roman engineering, these are nevertheless siege and counter-siege technologies relying on active offense and defense. Altough the use of sieges machinery by Gauls is anecotal (Vercingetorix used undescribed machines to break off the inner Roman wall during the Siege of Alesia) and Gaulish engineering was "limited" to wall construction, repairs and probably works in campaigns (such as bridge-building), this was an important change to be noticed and probably to be associated with the hypothetised military revolution in late independent Gaul, especially with the re-appearance of fortified sites in western Europe known as oppida by the IInd century BCE with what it implied : growing focus on these as economical, political and religious centers and giving their omnipresence, an important and more broad presence in Gaulish warfare.

Altough the categorization proposed by Stefan Fichtle is based on these, with significant arguments in favour of their ostentatory and symbolical function and at least a debate over their military efficiency, this debate is much less one-sided against efficiency than understanding it as complementary of the other functions than it used to be (at least in French academia) : signs of the emergence of Gaulish petty-states as their center and display of power as much as fortified sites against rival states but as well waves of migrations from across the Rhine (especially Cimbri and Teutoni). Their tactical weakness to Roman poliorcetics would not came from an inherent incapacity to think the siege warfare, but indigenous siege warfare (involving a series of sorties, thus many gates, a mobile defense on the walls, preventing fires and sapes) being mostly inadapted to Roman poliorcetics as Caesar entered Gaul in 58 BCE. Even there, the sieges of Avaricum, Gergovia and Alesia points how Gauls could adapt these into a broader strategical tought : their defeat sound the death knell of any ulterior development even as Romans used indigenous techniques to first fortify the Rhenish limes.

Long story short, the absence of siege engineering is mostly explainable trough an absence of need, or felt need, in indigenous warfare. But it doesn't imply the lack of siege technologies adapted to this context even if the brutal comparison with Roman siege warfare spelled its doom.

Jean-Louis Brunaux, Bernard Lambot; Armement et guerre chez les Gaulois, 450 - 52 av J.C.; ; Editions Errance : Collection des Hespérides; 1987

• Alain Deyber ,Les Gaulois en guerre - Stratégies, tactiques et techniquesr;Editions Errance; 2009
• Alain Deyber; Oppida en action, Le rôle militaire de l'oppidum gaulois (58-51 a.C.) in L’Âge du Fer en Europe - Mélanges offerts à Olivier Buchenschutz; (dir.)Sophie Krausz, Anne Colin, Katherine Gruel, Ian Ralston, Thierry Dechezleprêtre, Éditions Ausonius, Collection Mémoires (32), Bordeaux, 2013, p. 661-670
• Stephan Fichtl, Murus et pomerium : réflexions sur la fonction des remparts protohistoriques , Revue archéologique du Centre de la France [Online], Tome 44 | 2005,
• Pierre Moret; The purpose of Gallic oppida ramparts: a reappraisal; dans Ariane Ballmer, Manuel Fernandez-Gštz, Dirk Paul Mielke (ed.), Concepts of fortifications in Pre- and Protohistory, Oxbow; 2017