Archer Towers

Hello, I read through this thread and couldn't find the answer to my question: how do you upgrade castles? Just half an hour ago, I was playing Heroes, and a situation arose where the computer captured my city (the towers fired once), but when I attacked it, these towers started firing 5 times each. :confused: In general, can someone explain it to me? I would be very grateful. P.S. When I was attacked, there were only 3 units in the city, and only a few of each.