The attractions of Ayuthaya are the ruins of temples and palaces, and further there is (according to the Lonely Planet) nothing to do. The first is certainly true, the second a little bit.
The ruins are quite touristy, busloads of tourists arrive during the day, but most do not go beyond Wat Phra Si Sanphet. So in most ruins you can quietly look around, and even at Wat Phra Si Sanphet that is often possible. At every ruin you have to pay your admission, but if you keep your ticket you can usually visit all ruins without additional pay. As usual in Thailand foreigners pay more than Thai people (30 bath and 10 bath), and that applies to everything, not only for the entry fee.
In the evening some ruins illuminated, definitely worth it. At the corner of Thanon Chee Kun and Thanon Naresuan (on the quiet corner!) is a restaurant (sorry, we forgot the name), where you can eat well and where you have a beautiful view on a few illuminated ruins.
When we write about a place, we write about what we saw and did there, and what we think about it; no more and no less.
This is not a travelguide (we are not complete), and it is neither an advice to visit or not to visit a place. Other sites can give you that information.
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It is our description of a situation on a certain moment, influenced by our mood.