@Bubble99:
A post where you are bombarded by all of 13 consecutive images - furnished with no explanatory text whatsoever placing them, or, putting them, in any sort of a context - is exhausting to look at, one's eyes glaze over, for it is utterly over-whelming; why not break down the images into two or three separate posts?
While it has been said that an image is worth a thousand words, this is only true if the context of the picture is clearly understood in the absence of words, which is not the case here.
Moreover, adding models and pictures, paintings, of buildings (rather than photos of the actual buildings themselves) in your original post are redundant; as you have alredy provided more than enough examples in your initial post, they are not necessary and are a form of visual over-load, especially when squashed into a single post.
Now, you have provided no images whatsoever of either what you refer to as "Celtic" homes (and, by the way, what - or where - or when - exactly is that? Ireland? Scotland? Brittany? Wales? What do you mean by "Celtic"?).
And, for that matter, there are no examples whatsoever provided of Viking homes.
For a post that comes complete with such visual over-load, there are no examples whatsoever of half of what you refer to in your thread title.
@Longkeg, in his or her reply, makes some excellent points.
Roman culture, or civilisation, or - for that matter - occupation - did not extend to most of what is loosely described as the "Celtic" world, (or culture), and to almost none of the Norse world; thus, the architecture and the knowledge of - and mastery of - the building skills of the Roman world (which had been already degraded during the long, slow, collapse or decline of the western Roman Empire over the previous few centuries) were either lost, or had never been known.
That meant that the ability to construct the sort of buildings and structures - such as the vast public representational buildings - that had been the norm throughout the Roman world was lost, or didn't exist, or wasn't used for private dwellings; castles were built, but they were for safety, security and defence; cathedrals and churches were built - even in "Celtic" times, but they were for worship.
Add to that the locally available materials (stone, wood, etc) that one uses to build with; they are not the same in different parts of the world.
Moreover, to be able to build the sort of buildings of the Roman world requires a fair degree of wealth (brought about by conquest and/or trade) - which was absent in both "Celtic" and Viking society, which were neither secure nor stable - for successful extensive trade needs a safe and secure and stable and relatively well ordered - and well regulated - world in order for it to be able to take place safely and securely.
In order to be able to facilitate a world where trade can take place, on land, you need a world of roads, - and you need to be able to build roads, and ensure that these roads that are kept in good condition for travel and are deemed safe enough for travel, and that, in turn, implies the presence of, or existence of, a recognised central authority, in other words, a government, that can regulate - or has the power and wealth to be able to regulate and enforce such regulations - such things, or a strong regional authority.
For facilitating trade by water, this also implies societies wealthy enough to be able to build (and maintain, and repair) ships that can cross the seas or navigate rivers, ships that aren't used for naval warfare, or for attack and defence, or piracy, or raids, but for trade; and that requires a skilled, sophisticated, society, a sufficiently wealthy and safe workforce, - for ship-building is expensive, of people, time, resources - and usually an urban one, as well.
Above all, the sort of wealth - and the trade and craft specialisation - which creates that sort of wealth - and which allows for construction of comfortable buildings is only ever found in an affluent and urban society; merchants work best in relatively well off cities or towns, cities or towns where social mobility is permitted, where the exchange of monies, goods, ideas occur, where it is safe enough - and regulated enough - (commonly agreed and enforced and recognised and verified weights and measures of goods, and monies and minting) for markets to take place and to be allowed to, and able to, trade freely.
All of this implies a fair degree of literacy, numeracy, and urban spaces which are not just the headquarters of an occupying army and/or tax-collecting bureaucracy (though they can start out as that) but which can also function as places and spaces where exchanges (of goods, products, monies, ideas take place and where people can meet others).
Many, if not most, of these conditions weren't met, or didn't exist, in "Celtic" or Viking, times.
Now, of the medieval buildings in the original post:
Firstly, the vast majority are urban.
Secondly, the vast majority would be classed as "late" medieval - thus, these are the result or culmination of decades and centuries of acquired (and applied) building techniques and construction knowledge.
And, as
@Longkeg and
@DeltaMac have already both pointed out, local conditions (and locally available materials) would also drive architectural possibilities and regional preferences in determining what would be, and what could be, built.
Thirdly, none of those images depict anything other than the houses or homes of the comfortably off: These are merchant dwellings, or the homes of the fairly affluent; these are not the homes of the peasantry, or, of serfs, those tied to the land by custom and law in brutally unfair exchanges, and nor are they the homes of the urban poor; these are not the medieval equivalent of an urban slum.
In other words, even the buildings you depict are not representative of the era's 'style'; rather, they are representative of an urban, middle-class, late medieval style found mainly in affluent and urban parts of northern Europe during the era in question, or under discussion.
