The UNESCO World Heritage have listed to date, Five  World Heritage properties/sites in the Philippines. However, there are Four  World Heritage Sites listed in just one Category [the Baroque Churches in the Philippines Category] which technically means that there actually Eight World Heritage Sites in the country.
Of the Eight, Three are located in the Ilocos Province. Two in Ilocos Sur and One in Ilocos Norte. First listed in 1993 were the the Sta. Maria Church [also known as the Church of the Assumption] located in Sta. Maria, Ilocos Sur, and the Paoay Church [also known as San Agustin Church] located in Paoay, Ilocos Norte. They were listed together with Manila Cathedral and Miag-ao Church in Iloilo City.
The Sta. Maria Church, is built on top of a hill giving visitors a panoramic view of the town, the green fields and the blue sea in the horizon. One can reach the Baroque church via an 82-steps stairway. Two huge columns flank the church façade. What makes the church attractive is its reddish exterior due to the exposed brickwork. An octagonal bell tower is located nearby.[Source]
The Paoay Church is sometimes called “Severe Earthquake Baroque” for reasons obvious to anyone looking upon it. It has buttresses that extend out considerably from the exterior walls and provides a visual experience that is three-dimensional, unlike most of the churches in the country where the inherent beauty of the church is limited only at the facade. [Source]
For Unesco, “these [baroque] churches, the first of which was built by the Spanish in the late 16th century are listed as world heritage because of their unique architectural style as a reinterpretation of European Baroque by Chinese and Philippine craftsmen.”
Six years after, in 1999, the historic City of Vigan was listed next as world heritage in the Province. Unesco described the City as being “established in the 16th century, Vigan is the best-preserved example of a planned Spanish colonial town in Asia. Its architecture reflects the coming together of cultural elements from elsewhere in the Philippines, from China and from Europe, resulting in a culture and townscape that have no parallel anywhere in East and South-East Asia.”
Further, Unesco justified the inscription of the City in the world heritage list being a “representation of a unique fusion of Asian building design and construction with European colonial architecture and planning. Vigan is an exceptionally intact and well preserved example of a European trading town in East and South-East Asia.”
So, if you plan to go to Ilocos this Summer, visiting these World Heritage Sites in the Province should be a must in your Itinerary!
Postscript: The other World Heritage Sites in the Philippines are: The Tubbataha Reefs Natural Park [Palawan]; The Rice Terraces of the Philippines [Cordilleras]; and the Puerto-Princesa Subterranean River National Park[Palawan]