A flak emplacement and a command bunker were opened up by volunteers from Festung Guernsey.
Before the open day started at 2pm, islanders were walking up the hill to the bunkers, keen to see what was inside.
By the end of the afternoon, around 600 people had had a look around.
Project co-ordinator Paul Bourgaize said work had started to clear the flak battery last November, with three days of excavation. Around 500 tons of material were removed.
It then took five or six weekends to build a dry stone wall to hold back the earth and then a couple of more weekends to install new concrete steps to access the battery.
Mr Bougaize said the volunteers were glad to let the public see inside for the first time.
‘We talk to people too who turn up while we were working, but it’s nice to have a day where we can show people what there is and they can have a look around,’ he said.
Among the crowds were Geoff and Val Dumaresq. Mr Dumaresq was born during the Second World War and the couple can remember collecting items like German helmets and bayonets when they were growing up.
‘It’s a good thing to remember, because it is history,’ Mr Dumaresq said.
Andrew Kelly took part in the Fort Hommet heritage trail over Easter and was pleased to have the chance to look around the L’Ancresse fortifications this weekend.
‘It’s a good chance to investigate,’ he said.
‘We like to support Festung Guernsey. Their work is fabulous.’
When the group opened up the flak battery, the structure was in quite a good condition, apart from some rubble on the floor.
That meant islanders could see the stencilled instructions on the walls and the remains of the old electrical system.
Shirley Vaudin said looking around was very interesting.
‘We live locally and my husband plays golf, so it’s interesting to look around,’ she said.
Sam Sexton and her three-year-old son, Beau Martel, also enjoyed the afternoon.
‘We just go exploring and this is the history of Guernsey,’ she said.
‘It’s important that Beau knows the history of where he lives.’
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