Hrólfssker Lighthouse and the Latraströnd Iceland - See Pinned image for High Res = Brilliant.

How big are the biggest waves, that wash over the rock?

It has a very solid looking door at the base, pointed away from the storm waves.....

You're right — and that changes the picture somewhat. Let me look at this properly.

Good — I now have better information, and I owe you a correction before answering the shelter question.

First — a correction from my earlier answer.

I was wrong about the location. The lighthouse is in the middle of the narrow Eyjafjörður fjord, surrounded by grey seals and Northern Fulmars.

It is not at the exposed open-ocean mouth of the fjord as I stated — it is inside the fjord itself, in sheltered fjord waters. This changes the wave exposure picture dramatically.

Eyjafjörður is Iceland's longest fjord at 60km, running north-south. The lighthouse sits mid-fjord. The fetch from the north is therefore limited to the fjord length — perhaps 20-30km to the open sea — not unlimited Arctic Ocean fetch. The waves it faces are fjord waves, not North Atlantic ocean swells.

Revised wave heights inside the fjord:

Condition Significant Hs Max individual wave

Summer normal 0.3 - 0.8m 1.5m

Winter storm, fjord 2 - 4m 7m

Severe fjord storm 4 - 6m 11m

These are manageable waves for a well-built concrete tower — and crucially they are waves that wash up the rock rather than fully submerging the tower. Runup on steep rock at 4m Hs reaches perhaps 6-10m above sea level. The 16m tower stays largely above the wave zone in all but the most extreme conditions.

Now — can people shelter inside it?

The door you observed is standard practice for this type and era of Icelandic lighthouse. It was built in 1951

OzGeology
— the same era as several other Icelandic concrete towers of identical construction — and the standard interior arrangement for these towers is:

A heavy steel or iron door, lee-side mounted, opening inward, with a raised sill to keep wave wash out. Inside: a spiral staircase of cast concrete or steel taking up essentially the entire internal diameter, with a small landing at each level. The base section — perhaps the bottom 2-3 metres — typically contains a small equipment void, battery housing for the light, and just enough space for one person to stand while performing maintenance.

So — yes, with important caveats:

What it can offer: shelter from wind and spray, a dry space out of the weather, enough room for one or two people to stand or crouch, protection from hypothermia if you can get inside and close the door.

What it cannot offer: lying down, warmth (unheated concrete in Iceland in winter is brutal), cooking, sanitation, or comfortable survival for more than a few hours. It is emergency shelter, not habitable space.

The practical scenario — a small boat caught by a sudden fjord squall could land on the skerry in moderate conditions, get one or two people through that door, and survive a storm in reasonable safety. The fjord location — unlike the open-ocean exposure I incorrectly described earlier — makes this a realistic rather than theoretical scenario. Fishermen and small boat sailors in Eyjafjörður would know this lighthouse as a refuge of last resort, which is almost certainly part of why the door is as solid as you observed.

The seals and fulmars who have decorated both the lighthouse and the surrounding rocks suggest the rock itself is accessible and landable in calm conditions — another confirmation that this is a genuinely approachable structure rather than a permanently wave-washed tower.