If you are member of guild in the town an if you reached "ratsherr"-level, you can ask in town-hall for building out the wall. You will get to win the election, if you have more than two factories out of the wall. But the town will only build the wall if there is enought material on market.
The town builds eyery 8 days until 10 wall-sections. If i remember correctly you need 1 iron, 1 wood and 10 bricks for one section. The material should be on market in the morning of wall-building-day. Roland postet in 2006 the date of wall-buildings:
Dafür kann ich die Baudaten bieten:
01.01;09.01;17.01;25.01;02.02;10.02;18.02;26.02;06.03;14.03;22.03;30.03;07.04;15.04;23.04;01.05;09.05;17.05;25.05;02.06;10.06;18.06;26.06 04.07;12.07;20.07;28.07;05.08;13.08;21.08;29.08;06.09;14.09;22.09;30.09;08.10;16.10;24.10;01.11;09.11;17.11;25.11;03.12;11.12;19.12;27.12; und dann wieder von vorne, also Start mit 01.01.