How do i gain resideny in belgium with my fiancee being a belgian citi

Hi Dave,
I am not really sure about your case, but i would like to share what i know and my experience right now.
I am from Indonesia, coming to Belgium with my boyfriend using Schengen Visa (family and friend visit). and we're prepared to marry here.

From what i know from the city hall in Ath, marriage is a human right, so whoever can apply for marriage, and i believe even for you (in this case you are illegal here) that will be fine as long as you have all the documents. the documents that required from the city hall here are:
1. Birth certificate
2. Celibacy certificate
3. Residency certificate
(all the 3 documents above must be legalized by the Belgium embassy in my country, then translated, and the translated document must be legalized again by justice here)
4. proof of nationality, issued by embassy of my Country in Belgium
5. De Coutum certificate (its a certificate that explain the rule of marriage that valid based on the law in my country), so this certificate also issued by embassy of my Country in Belgium.

After all the documents ready you can apply for marriage and normally you will have like 1 or 2 months interrogation, and during that months you are legally stay and protected by local government.

and i think after the marriage, you still have to apply for living in Belgium, and for that your spouse which will be your sponsor must have job and proof of the financial that can support your living and she also has to have flat, etc.

anyway, these information is based on the City hall in Ath, and also my understanding. So, good luck for you