The 64-year-old man from Llandovery, who has asked not to be named, was working on the refurbishment of an accommodation block at the site when he fell three metres on 15 March 2012.
He lost consciousness and suffered bleeding to his brain, a fractured cheek bone, bruised ribs and further damage to an existing back condition. He now requires morphine for his back pain and has been unable to return to work.
Evans Brothers Builders Ltd, the principal contractor overseeing the project, was prosecuted by the Health & Safety Executive (HSE).
Carmarthen Magistrates' Court was told that the company had arranged for the scaffolding to be removed from the site before the injured worker had finished fitting new waste pipes to the outside of the building. He was given a scaffolding tower to use but it was found to have been in a dangerous condition, lacking guardrails and internal ladder.
Evans Brothers Builders Ltd pleaded guilty to a breach of the Work at Height Regulations 2005 by failing to ensure the work was properly planned, appropriately supervised and carried out safely.
The company, of Meadows Bridge in Parc Menter, Cross Hands, was fined £5,000 and ordered to pay £3,685 in prosecution costs.
Speaking after the hearing, HSE inspector Anne Marie Orrells said: "The worker sustained life-threatening injuries in the fall and could easily have been killed. His injuries will affect him for the rest of his life.
"If Evans Brothers Builders had planned the project properly then it would have known it needed to keep the scaffolding for a few more days, or provide alternative equipment so that the waste pipes could be fitted safely.
"The scaffolding tower and unsecured ladder it provided was clearly not fit for use and the moment the worker attempted to climb it, his life was put in danger."