With the expansion of many cities, at times halachic questions arise as to which day is the proper day for celebrating Purim. Regular cities celebrate on the 14th of Adar, but cities that used to be walled celebrate on the 15th. That is generally thought of as Jerusalem, as the Old City of Jerusalem is walled and everyone knows about it. The truth is though that numerous cities used to be walled, but today's modern cities are not generally in the same exact place, and the original walls do not generally exist for us all to see and remember. Yet, sometimes a city expands and bumps up to the original location of what used to be the original walled city, and that's when the questions arise.

This has now happened in Ashdod. A delegation went to ask the posek Rav Mordechai Gross if in Ashdod they need to celebrate on the 14th or 15th of Adar. With the expansion of the city and new neighbors built in proximity to the biblical Ashdod, they were unsure what to do.

Rav Gross paskened that while they celebrate on the 14th, they should be careful to also read the megilla, without a bracha, on the 15th, just in case. Rav Gross said the area of some of the new neighborhoods might be not just near the ancient city but right in the same location. Because of the uncertainty, it should be read on the 15th as well.
source: Kikar

I will just point out that Bet Shemesh is in a similar situation. I know some people who have been reading on the 15th for many years already because of the proximity to both Tel Bet Shemesh and to Yarmouth. Now that RBS C (Gimmel) has been built right on Yarmouth, there is an even stronger argument for reading on the 15th.