They are not free to go anywhere. Even many Muslim were killed by Hindus for not accepting their religion.
So, the purpose of creation of Pakistan is to create and build such community where Muslims are free and they can spend their life according to their Holly religion Islam. At last, Pakistan came into existence on 14 august 1947 by the great efforts of Mohammad Ali Jinnah
. As creation of Pakistan is based on Islam, then there would be each and every thing according to Islam. The rules and regulations that will be made should be according to Islam and many points are included in the constitution of Pakistan as well. For example, as we know that the head of Muslim state should be a Muslim and that point is also included in the constitution of Pakistan that is the president of Pakistan should be Muslim.
So, we can say that Pakistan is a fortress of Islam because Islam provide base in the creation of Pakistan and it is possible only due to struggle and hard work of Mohammad Ali Jinnah.