Can I eat it only in Ayutthaya?
No. It is a dish eaten in other cities too. In Ayutthaya you could choose it as a meal between visits to the ruins.
Kuai Tiao Ruea · ก๋วยเตี๋ยวเรือ
If a bowl of noodles sounds good while you tour the ruins in Ayutthaya, try boat noodles. Noodles and meat are served in broth, and together with grilled river prawns it is famous in Ayutthaya. Choose the noodles and meat from the menu and enjoy it with the broth while it is warm. It is eaten in other cities too, but it is a good meal to choose on an Ayutthaya trip.

It is a dish in which you enjoy broth, noodles and meat together. The taste of the broth can change with the meat and seasoning. Try it first without adding seasoning, then add chilli or other seasoning a little at a time.
Order by looking at the noodle type, the meat and the bowl size.
On a food trip in Ayutthaya you can meet boat noodles, grilled river prawns and roti sai mai together. A plan of a noodle meal at lunch and roti sai mai as a snack also works well. But boat noodles can be eaten in places other than Ayutthaya too.
No. It is a dish eaten in other cities too. In Ayutthaya you could choose it as a meal between visits to the ruins.
No. The meat, offal, seasonings and broth differ by menu item. If there is an ingredient you avoid, ask whether it is also used in the broth rather than only asking to leave out the meat.
If you want to read more about these dishes and regions, these sources are a good place to start. Ingredients and cooking methods can differ from one eatery to another.
Sharing image credit: Flickr user Alpha · CC BY-SA 2.0