Protesters used a wire basket to haul lumps of masonry on to a 50ft gantry above the main entrance of Dale Farm yesterday.
The heavily fortified entrance sits on the only road into the camp and is the likely route bulldozers will take on eviction day.
Police fear that the watchtower may be a focal point for violence during evictions at the camp in Crays Hill, Essex.
Publicly, the activists insist that the rocks and bricks are simply being used to hold down the tarpaulin that surrounds the shelter and would not be used for violence.
However, an insider at the camp claimed that they would be used to pelt bailiffs and police if they appeared to be getting through the defences.
Gathering: In a close up of the above picture, rocks can clearly be seen
‘Rocks have been used by anarchists against police to good effect in the past at previous demonstrations and the same will happen again here if necessary,’ the source said.