Blockade is and option for "attack" a town.
you can send a few troops (no siege machines needed) to occupy a square next to the enemy (or suspended/inactive) player's town and start a blockade.
Steps to make a success blockade:
-Research the Blockade technology in the military page of the "Research Tree".
-Make an army (could be made from 2 to 100k troops).
-Select the option "send army" of the town targeted.
Select the option "Blockade" and mark the number of days, hours and minutes for occupy the town.
-Select the square near to the town where you want to make the encampment.
and done!!!
The point of doing a blockade is that you're not attacking the city, you just put an army outside the town that blocks all the caravans that go and arrive to the town and take all the resources that they were carrying.
This is a great attack strategy for big cities where you have now idea of how big is their army and also block all the trade movements of that town so they can't progress.