I just attacked my first ruin and this is what I found
1) Send your explorer out to explore the squares in your area. Your explorer will send back a report on what he finds. Be sure to look at the details before you delete it. If there is a ruin there your explorer's report will tell you and it will also show up on your map/grid and it tells you what level the ruin is.
2) Ruins that are in your area will also send monsters to attack your town once in a while (if your gate is open). Your message will tell you that a Robbery is on the way. It also tells you how many monsters are coming and where they came from. So if you have not already found that ruin by exploring you will now have the coordinates of where its at and can send your explorer to that square to see what level it is.
3) Before you go to attack the ruin send your spies into the ruin to find out what's in there. The spy report will tell you how many monsters are in there and what level they are. It also tells you how many resources are there to steal.
4) If you do not have enough military units to attack it right away - be sure you send spies in again before you do attack because the monsters keep producing a certain amount every day. So it gets harder to attack them the longer you wait. You can tell how many monsters a day it produces by scrolling over the main ruin building in the center.
5) Be sure you have enough space in your storage to store all the goods you get from the Ruin before you attack it, so you do not loose all the goodies
6) Once you attack and defeat the Ruin monsters and steal the stuff it will no longer be there and disappear.
I hope this helps and sorry I cannot help you as far as military strategy because I am new also and just learning
