Good strategist can use defensive troops against siege!
Siege armies move slowly so you have a lot of time to prepare.
Step 1: Scout attacking town with advanced scouts, to see army composition.
Step 2: dodge any direct attack on town (most players will try to wipe out any forces in defending town before siege lands)
Step 3: When siege army hits the town it will need around 10 minutes to set up camp. In that time you should send out your defenders to most likely siege position, based on scouting information. Your defenders move faster than siege army so they will be there first (occupy for 10 minutes). Usually there aren't many good siege positions, but if there are you can always split your army. With new map arrow movement indicators you may actually see where siege camp is going to land. I haven't tested this map arrow hypothesis though.
Inexperienced attacker will just sent siege army with many defensive forces and will not hit siege position with cavalry/infantry few seconds before siege lands. Such an attacker is easy prey for defensive forces...
But primary role for good defense force is to actually siege yourself. Good defenders are most dangerous in this game, since siege is ultimate attack and you need many defenders for sieging...