I believe there is a table on the forum, probably on the data base section. You would need to search for it 

However the information can be found in another form by opening the troops training page, by opening each type of troop you will find which other types it is strong or weak against.

Example Melee is strong against Cavalry, but weak against Siege, and medium against Ranged and Killer, but there are other factors you might like to consider when choosing, like cost, how much they carry and how quick they move.