Polonnaruwa is a city in the North Central Province of Sri Lanka. The second most ancient of Sri Lanka's kingdoms, Polonnaruwa was first declared the capital city by King Vijayabahu I in the 11th century AD and maintained its status until the 13th century. In 1982 the ancient city of Polonnaruwa was inscribed on the UNESCO World Heritage List. The history of Polonnaruwa is available at this link 
What's nice about Polonnaruwa is that the ruins are in one compact area so you can visit them quite easily. Just one three wheeler ride from where you're staying to the entrance and you can walk about them all day long.
Polonnaruwa is 216 km or 134 miles from Colombo.
To travel there, you've got several options. They are, in order of preference, car, train and bus.
The car is the easiest and most expensive. Any taxi service will charge you around LKR 60 per km, so 13,000 LKR total. Use Pick Me or Uber and there's the option of a smaller car costing around 30% less. The advantage is that you will sit in air-conditioned luxury all the way to Polonnaruwa and the road is quite nice with plenty of greenery along the way. The old, massive trees tend to shade into the road once you leave the urban areas, creating a canopy effect. A trip is five to six hours inclusive of breaks for food, fruit and shopping (although shopping along the way is just overpriced and not recommended).
Train is next, though frankly, bus is kind of tied with it. It's quite cheap: you'll that find the three wheeler ride to the train station costs about as much. You get on the train to Trincomalee at Colombo Fort station (http://www.srilankatourism.org/lang/en/traveller/trainschedule.php), connect in Gal Oya and you're in Polonnaruwa. The trip'll take you a good part of a day and the train leaves the station at 6.15am so plan accordingly. It is not however an air conditioned experience but it is a train with all the scenery and Theroux-like hijinks that implies.
Bus is always my third option. Buses also leave from Colombo Fort but from a slightly different location. Take the intercity to Polonnaruwa. With luck and blessings from whatever deity you subscribe to, you should arrive in Polonnaruwa in six to eight hours. October 2015 Price is 250 Rup from Colombo to Polonnaruwa.
Bicycles are the best way to get around. Most guest houses will rent out bikes, but be sure of the condition when you pick it up. Check if the tyres are properly inflated etc. It's still possible to walk, because there's plenty of shade around the monuments. However be prepared for 6-10 kilometres of walk.
Just north of present-day Polonnaruwa town, 140km (90 miles) north of Kandy, are the ruins of ancient Polonnaruwa, which date from the late 10th century, when the Chola kings of southern India invaded Sri Lanka and conquered Anuradhapura.
The history of this ancient city is essentially connected with three Sinhalese kings. They are:
Vijayabahu-1 – King Vijayabahi-1 is the founder of this great city. The king was on the throne from 1055 AD to 1110 AD. King Vijayabahu-1 liberated the country from Chola dynasty, who ruled the island for a long time.
Parakramabahu – King Parakramabahu ruled the country after the king Vijayabahu. The country was ruled by King parakramabahu from 1153 AD to 1186 AD. The king Parakramabahu inherited the kingship from his grandfather King Viajayabahu-1. According to the historical information, King parakramabahu undertook the responsibility of restoring the religion of the island, which was neglected during the foreign rule. Same times he had undertaken the responsibility of improving the agriculture and economy of the country. The king was able to bring the entire country under his control. King Parakramabahu had invested a large sum of money on the infrastructure and built a large number of tanks, and canals in order to increase the agricultural production of the island.
Nissankamalla – King Nissankamalla became the king of Sri Lanka in 1187 AD was on the throne until 1196 AD.
Polonnaruwa is one of the must most important archaeological sites in Sri Lanka and many tourists visit the city during their holidays. Most monuments in Polonnaruwa are closely related to two religions, Buddhism and Hinduism.
Thuparama is the best preserved ancient construction in the city. The construction is situated in the quadrangle towards the northern end of the premises. There are six other ancient religious constructions in the quadrangle alone with Thuparama. The age of the construction is not known to anyone but archaeologists believe the construction took place during the medieval period.
As of September 2019, the site can be visited from 7 a.m. to 5:30 p.m. Entrance fee for Foreigner is 4500 LKR (or 25 USD, Mar 2019), tourists from SAARC countries pay half price, one must produce a passport to get the discount. The ticket is available at the Museum. Make sure to purchase a ticket before heading towards the ruins, else you need to go back to the museum as tickets are not available at the entrance. Also e-tickets can be prebought Heritage site e-tickets. Unfortunately, the round trip tickets for the cultural triangle (50 USD) are no longer available.
In this area, you can find plenty of wild life reserves. Try to find a reliable person and visit the places like "Minneriya", "Kawudulla"
Take a nice stroll at the ancient Bendiwewa reservoir, part of the Parakrama Samudra reservoir complex, which borders Polonnaruwa to the west. This shallow reservoir makes for a great picnic site, and during dry seasons may be shallow enough to allow traversing large sections of the lagoon by foot. The reservoir is surrounded by an electric fence but don't let that deter you - the fence is there to deter Elephants from entering the city, however common that may be.
There're dozens of guesthouses, mostly situated along Batticaloa road. price negociable if staying longer. usually starts from LKR 800, with WI-FI atleast LKR 1000.