When you live on an island, there’s always the chance some bugger’s going to come and try to take what’s yours.

In Britain, we spent a long time being invaded: Romans, Saxons, Vikings, all taking advantage of the fact we on the island had a bit of an identity crisis. Who exactly were we? We kept asking ourselves, as we hacked bits out of one another, and assimilated one another’s culture.

At one point we were so invaded we were all speaking French.

By the time Bluff King Hal was on the throne, we had a fair idea of who we were. We spoke English, we had had a monarch uniting our feuding parts for some centuries, and we had had some glorious moments when large bits of France were part of an empire ruled from England.

But King Henry VIII always had half an eye on those pesky invaders.

Which is why he built a series of what are known as ‘Henrician castles’ or ‘device forts’ along the South Coast of England.

Stark round military outposts, they are nevertheless Tudor Castles in every sense. They may have ten-foot- thick walls, and be round to cause arrows and cannonballs to glance off, but they have lead pipes redolent of Windsor Castle, and gorgeous carved wooden pillars, and have some of the Tudor romance which is simply not present in the squat round Napoleonic forts which also pepper the south coast, the Martello Towers.

Yesterday, we trotted off through what is a frankly puzzling landscape to visit just such a tower. Portland has its fairy tale elements, but the romance of Chesil Beach, a great long natural spit of shingle which spans the sea between Portland and mainland Dorset, comes to an abrupt end as you approach the little island. It is barren, with the look of one of our industrial estates. Yet right next to the water’s edge, adjacent to the abandoned heliport, sits Henry’s Tudor castle.

It doesn’t get much more incongruous than this.

However, we visited, and it was magical, and whilst peering from the cannon ports we noticed a golden retriever swimming in the sea next door.

The fort is flanked by two sandy beaches: the haunt of dog walkers far and wide. The cobbles of Chesil are not kind to dogs’ paws, and so owners come here, where their dogs can sniff and paddle and chew the odd bit of seaweed or boat rope for sport.Then and there, we resolved, we must bring Macaulay the Dog to visit the Henrician beach.

An hour later, the dog was knee deep in seaweed, his nose questing for the unmentionable. Finally he was off the lead in a place where he could actually walk. And immediately Macaulay became professionally busy, like a rudderless metal detector, sweeping the beach, becoming the seaside police.

He would leave no stone unturned.

The castle juts out into the sea, a curved interruption to the surrounding sand and marinas. You can go to its left or its right, but not both. Not, that is, unless you want to scale the treacherous seaweed -viscous cobbles and walk along a low stone lip which skirts the fort, wide enough for only tightrope walkers.

And accommodating, also, for small dogs.

I looked up from my pebble-appraising reverie to watch my dog’s behind receding rapidly around the lip of the fort.

Horrors. Would we ever get him back? I had already caught him sizing up military Tudor crevices. Small dogs like crevices. I was already listening, in my mind’s ear, to the Fire Department reading me the bill for his recovery.

I shouted his name like a howitzer, and he turned abruptly and pottered back along the lip to my side of the beach.

Minutes later, my concentration sadly lapsed, I looked up to watch the tail retreating towards the other side, where the seaweed is always greener. And smellier.

Macaulay! I hammered out.

But the dog had become temporarily, and profoundly, deaf. I watched him clatter round Henry VIII’s fort, out of sight, who knew where.

Curses.

Phil and I went to battle stations. You go round and find the other beach, I said, I’ll stay here in case he comes back. Phil strode off with grim determination towards the unknown.

I called, of course I did: and after about two minutes, the dog clattered back round as if nothing of import had happened.

“Phil,” I hollered, “The dog has come back!”

No answer.

I glared at the dog. This is all your fault, cur, I implied. The dog looked happily impervious. We set out after Phil, who is our other loose cannon.

If I didn’t get to him in time, he was going to check the beach: and then he would not bother coming the long way round. He would try teetering round the lip of the fort.

We hastened as fast as we could, following a route as convoluted as only Portland can supply, until we reached the other side.

And there, just beginning the treacherous balancing act on the lip of the fort, was my husband.

He jumped down. I breathed again. The dog ruminated happily.

Two more invaders repelled on a Sunday afternoon.