Some important  archaeological sites he has worked on are detailed here.

• A unique Buddhist image house , it is the only extant stone-built Buddha image house in Sri Lanka in the ‘gedige’ style of the Pallava Architecture of the 7th century AD, of Mahabalipuram, South India.
• Nalanda Gedige is entirely made of stone, differentiating it from other Sri Lankan monuments of the gedige style which were brick-built. 
• The monument, dated between 8th-10th centuries AD, displays a synthesis between Theravada and Mahayana forms of worship and between Buddhism & Hinduism.
• It holds a central and strategic position almost equidistant from Anuradhapura, Polonnaruwa and Kandy.
• It is the site of ruins of an ancient monastery; what remains intact is the Gedige Shrine and an adjacent small Dagoba on a terrace.
• Some previous conservation of the site had been done at its original location, under distinguished archaeologists H.C.P Bell and Prof. Senarat Paranavitana.
• In the 1970s, the hydropower project of Mahaweli river threatened to submerge this monument. Hence it had to be preserved at a higher elevation. The situation that faced restorers was similar to that of Nagarjunakonda in India, or Abu Simbel in Egypt.
• Prof. Prematilleke was appointed to direct the restoration project. He was adamant that this shrine should not be moved from its important location, only raised at its original location to avoid submersion. 
• It was then decided to dismantle the whole structure and reassemble it at a higher elevation, in the same location. It was re-established about 23 feet above its previous elevation.
• The work was carried out under his direction by a team of workers led by W.H. Wimaladasa, an experienced stone-mason of the Archaeological Survey. Prof. Prematilleke included Archaeology students from the University of Peradeniya in this work, giving them an invaluable on-site experience.
• A Bodhisattva figure which was previously mistaken for a guardstone due to its shape, was identified correctly by Prof. Prematilleke, and replaced in its rightful place inside the shrine room of the Gedige.
• The rescue of the Nalanda Gedige shrine goes down in history  as a feat similar to that of Abu Simbel in Egypt. The restoration of the Gedige goes a step further in having used manual labour and basic methodology in place of advanced technology (which was unavailable).
• Raymond and Bridget Allchin, renowned British archaeologists, commented thus on the restoration: ‘…that in the process of restoration, so much care and research should go into the errors of previous attempts of reconstruction, and thus restoring the building, as far as perhaps will ever be possible, to its original form is most exciting‘ (from Nalanda, a Short Guide to the ‘Gedige’ Shrine by P.L. Prematilleke)

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Alahana Parivena, Polonnaruwa (1981-1999)

• Prof. Prematilleke was the Director of this project.
• When the project commenced (on 2nd April 1981), he worked together with Prof. Janice Stargardt (link to collaborations page) from the University of Cambridge.
• He not just directed but also actively participated in the excavations, worked in close collaboration with conservation experts, and oversaw analysis and documentation.
• The site covered more than 200 acres of land. Including important monuments such as the Menik Vehera, Rankot Vehera, Southern monastery, and many other Buddhist and Hindu shrines. 
• It was a seat of monastic learning, built in the 12th Century by King Parakramabahu the first. It is said to be built on a cremation ground hence the name Alahana Parivena (crematory monastery).
• This project was undertaken in order to understand the contribution of this institution to society at that time.
• “Prof. Prematilleke and his students “camped” in a clubhouse there, began a chummery, employed the villagers as labourers on the site and set to work.” Source: Surgery in an ancient kingdom (sundaytimes.lk)
• Major activities done under the project were excavation, conservation, landscaping and documentation. Prof. Prematilleke upgraded signage and information at the site.
• The monastery consisted of many separate units, each unit had its own living cells and several seem to have a shared common bath house and other facilities for monks. On the highest terrace, stands the remains of the Buddha Seema Pasada (chapter house).
• The main new discovery in the excavated area was a monastic hospital for monks, which was unearthed in 1982. It had an inner residential area, and an external section for treatment purposes with a medicinal trough. It is, importantly, the only ancient hospital in the world where instruments were found within the hospital itself. 
• Prof. Arjuna Aluvihare, Professor of Surgery, University of Peradeniya was consulted on the surgical instruments found, and he co-authored many publications and presentations on this with Prof. Prematilleke. For further information: click here 
• Prof. Eugene Wickramanayake, Professor of Anatomy, University of Peradeniya (link to collaborations page) and Prof. K. A. R. Kennedy, a renowned anthropologist from Cornell University USA, were consulted regarding the remains of a skeleton that was unearthed at the excavation site, carbon dated to historic times.  Prof Kennedy held an on-site workshop in 1987 on the same.
• He researched the Vatadage at Polonnaruva from 1983-85 with Prof. Takeshi Nakagawa of  Waseda University, Tokyo.
• In 1988 he collaborated with Mr. Li Tsiang and a team of other experts from the Beijing National Museum on the Chinese Ceramics discovered. They also held an on-site workshop.

• Two archaeological techniques used for the first time in Sri Lanka for recording purposes:
  • Photogrammetry – a scaffolding was placed over an excavation pit, to take aerial photographs to record the depth of stratospheric layers

• • Photogrammetry – a scaffolding was placed over an excavation pit, to take aerial photographs to record the depth of stratospheric layers

• Artifacts unearthed as well as replicas created by local artisans under the guidance of Prof. Prematilleke of the original state of important buildings in the Polonnaruva archaeological site area, were preserved in an on-site museum created by Prof. Prematilleke.
• Three other major monastic edifices were also conserved under this project – the Tivanka Image House, Damila Thupa mound (seen here before restoration) and Gal Vihara monastery.

• The ancient city of Polonnaruva was named a World Heritage Site by UNESCO following a report submitted by Prof Prematilleke and others.

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Kandy Project (1981- 1999)

• Prof. Prematilleke was the Director of the project.
• Its objective was to recreate and protect the environs of the Sri Dalada Maligawa, 
• The palace complex was investigated including the Royal Palace, the Audience Hall, the Queen’s chamber (Medavasala), The King’s Harem (Pallevasala), The Queen’s Bath (Ulpenge) and Jayatilake Mandapaya. The Natha and Vishnu devales were also investigated. 
• The image house area of the Nagavimana in the Malwatta monastery area was also excavated. Some significant structures were found.
• Structures and artifacts of significance were found in the Royal Palace, Terrace mound east of the Palace, Queen’s Chamber, Queen’s Bath, and Natha Devala premises. The original foundation levels of the Natha devale and and Dagaba No:1. Artifacts found included stone sculptures, Buddha statuettes, parts of a large seated Bronze Buddha statue (which if complete, would have been the largest Bronze Buddha statue ever found in Sri Lanka), local and foreign ceramics, glassware, metalware, coins, bangles, beads, cannonballs, iron slabs, wax blocks, copper plaques etc.
• Artifacts that were found were conserved and treated at a mini laboratory that was organized by the Kandy Project.

• Prof. Prematilleke was the team leader of this massive project undertaken to prepare a detailed report on all the archeological monuments found around the entire Sri Lankan coast. After travelling for research, the writing of the report was done with the entire team hosted at his Kandy residence!
• Objectives were:
  1. To review the present status of the sites identified by the Revised National Coastal Zone Management Plan of 1997.
  2. To conduct field investigations and update inventory of high priority archaeological, historic, cultural sites and sites of natural significance.
• Districts that underwent the most extensive investigation included Ampara, Batticaloa, Trincomalee, Mullativu, Jaffna, and Mannar.

ICOMOS World Heritage Site Monitoring Mission – 1994 November-December

• In early 1993, Sri Lankan authorities invited ICOMOS to collaborate with officials in Sri Lanka to carry out monitoring missions for 3 World Heritage sites – Anuradhapura, Polonnaruva and Sigiriya.
• Prof. Prematilleke was the Sri Lankan archaeologist included in the  team that undertook this mission, which comprised local and foreign experts.
• Objectives were:
  1. To provide advice to Sri Lanka concerning significant conservation and management issues at the sites.
  2. To feed into a regional monitoring process to assist the world heritage committee, UNESCO and others for development of regional conservation programmes.
  3. To test the methodology used during the mission to advise UNESCO / ICOMOS concerning elements and approaches appropriate for application elsewhere.

Excavation of Bodhi Shrine at Galmaduwa, Kundasale, Kandy, 1979

This was a training course organized by him for Archaeology Students of Peradeniya.

Field survey of sites – Victoria Dam project, 1979

He conducted a preliminary survey of the Gonawatta Temple, Gurudeniya in the area getting submerged by the Victoria Dam project, together with students from Peradeniya, and submitted a plan for reinstallation of the image house in a cave at higher level.

Two important foreign sites he was consulted on were

• Lumbini (Nepal)
• Fua Malak (Maldives)