It’s hard to think of two building projects less alike than Portmeirion, the whimsical Italianate holiday village that was the career-long labour of love of the architect Clough Williams-Ellis, and the theatre and student housing tower of Coleg Harlech, a rugged work of concrete brutalism. Yet they were completed within a few years of each other, 1975 and 1973, a few miles apart on the coast of north-west Wales. A good way to experience the poles of British 20th-century architecture is to walk from one to the other, and enjoy some ravishing landscapes on the way.

This is a land that runs to extremes of large and small, and harsh and sweet. There are mountains, rising towards their highest point at Snowdon, and extra-terrestrial terrains sculpted by slate quarries, but also little cottages and wooded lanes, and the narrow-gauge Ffestiniog railway, whose minus-sized steam engines chug endearingly over dizzying valleys.

Portmeirion is at the pretty end of the spectrum. “As a child,” declared its creator Williams-Ellis, “I just lamented that my lot was cast in Victorian non-conformist Wales, instead of some such sparkling setting as decadent 18th-century Venice.” In 1925, he bought a scenic snippet of the coastline, where a steep cleft in the land descends to the broad sands of the Dwyryd Estuary, and set about bringing a bit of the Mediterranean to his home country.

For the next 50 years, with a break from 1939 to 1954, he created what stands there now, an artful assembly of turrets, domes and loggias, of scrolls and finials, blue-pink-turquoise-peach-ochre, with images of mermaids, cherubs and eagles and fragments of older buildings salvaged from demolition sites. The prevailing Italian spirit also accommodates touches of Scandinavia and Japan, and an Alpine or Hapsburg onion-domed tower. The industrial world is wished away – a single petrol pump (now antiquated but modern in its time) is surmounted by a plumed heraldic bust.

The village is avowedly theatrical. An imposing portico turns out to have nothing behind it, like a stage set, and illusionistic paintings of stone and carving mingle with the real things.

Living rock, which sometimes bursts through the delicate architecture, is juxtaposed with papery representations of it. Views are framed and managed, like pictures, by archways and openings. Portmeirion seems also to have drunk from the local Alice-in-Wonderland potion of shrinking and enlarging: optical tricks make buildings look bigger than they really are.

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It is brilliant and seductive, but also a bit weird. It has the Truman Show creepiness that comes from inhabiting someone else’s all-encompassing fantasy, for which reason it made the perfect location for The Prisoner, the cult 1960s TV series about an intelligence agent trapped in a mysterious village. Also contributing to its odd atmosphere are some quite terrible wall paintings, which show the lifeless cavorting of sexless nudes.

Coleg Harlech was built as an adult education college to the designs of the north Wales practice of Colwyn Foulkes & Partners. Its housing tower, a plain oblong standing like a sentinel at the foot of a sea-facing slope, has, as Owen Hatherley puts it in Modern Buildings in Britain, “the elemental power of the Welsh landscape”. It is finished in greenish-grey pebbly concrete, scored with exaggerated joints between the panels to give what Hatherley calls its “ascetic intensity”. The regular rhythm of its windows is interrupted by one off-beat move, a bridge that exploits the slope behind to make the main entrance high up the block, with a band of high windows that lights the shared areas to which it leads.

The theatre is more intricate and dramatic, with a curved-walled auditorium hovering above glassy foyers, and the vertical shafts of stair towers rising up the outside, a version of the constructivism of the early Soviet Union modified for the land of Owain Glyndŵr. In between the theatre and tower is Wern Fawr, a stony classical structure originally built as a house in the 1900s. They are a punchy and distinctive triptych.

In fairness to the reader, I must report that the current state of the Coleg Harlech buildings makes for an austere experience, one perhaps only for serious students of brutalism. The theatre and old house are under renovation, while the tower is currently disused and broken-windowed. If you’re looking for more conventional tourist attractions there’s the magnificent Unesco world heritage site of Harlech Castle, round-towered on a clifftop like one from a story book – as relentless and stark as the college, to be sure, but lent romance by the passage of centuries.

And then there’s the walk between the picturesque village and the brutalist college. You’re travelling along a geological border, where the high land gives way to coastal levels that were once submarine, so you get sweeping views of bodies of water and distant mountains, and winding routes through dense woods, hamlets and small fields. A lovely moment is the dragonfly-rich Llyn Tecwyn Isaf, reflecting trees and cloud-shadowed slopes in its glassy surface, that is everything you could want a lake to be. Exactly which route you take is up to you, but it’s better (where possible) to take the lanes and footpaths that loop inland than the less charming A-roads and B-roads to which Google maps will direct you.

It should also be noted that this part of Wales, if coming from other parts of Britain, is hard to reach by public transport, so it’s best to enjoy the journey and take your time. It’s possible, for example, to travel to the coastal town of Llandudno, then inland to Blaenau Ffestiniog, and then walk the 10 or so miles from there to Portmeirion. Or use the holidaymakers’ ride of the Ffestiniog railway as an actual transportation facility (if an expensive one) and travel on it part of the way.

This route offers more sights of the tough and the sublime. Blaenau Ffestiniog is the location of the area’s most dramatic slate quarries, and you’ll get views, while walking through wooded slopes and over rushing rivers, of the gigantic, decommissioned nuclear power station at Trawsfynydd, designed by the architect of Coventry Cathedral, Basil Spence. Wherever you go, it’s hard to go wrong in this part of the country, where fearless and singular buildings punctuate the majestic landscape.