Squatters are technically trespassers. The adverse possession laws in city limits pertain to adjacent land owners (not squatters). If the squatter wanted to get adverse possession they would probably have to adversely possess for 15 years. Also note the person seeking adverse possession must have color of title and the property covered is unimproved. In other words if you are a squatter enjoy your free rent while you can.
Every legal matter is fact specific, and there are often nuances in every case. This is intended for comment only, and does not create an attorney client relationship.
I agree with Mr. Hill. Additionally to "perfect" title, the squatter will need to file a case in which the judge will render an order "declaring" the squatter the owner of the property.
Actively practicing law in Texas. Inactive licenses in Arizona and Georgia. All answers are general in nature and no attorney/client relationship exists in this forum.