I read this book back in the 90s, I believe, but it could have been published earlier. It was a fantasy novel that begins in a city.
I think it starts with an assassin who was going to kill a king or some royal. The assassin kills some guards and sets up in an old towee. While waiting either a witch or elflike woman comes in and magically binds him. Not sure if there are magic chains or ice chains. There may have been children hiding that see the whole thing. I remember the author creating a sense of the assassins terror. This all occurs at the very start of the book, prologue or first chapter.
The assassins weren't the main characters. I think the assassins were the children (the witch may have been a child) or there were children who witnessed the assassin preparing for the hit, killing a couple of guards and then he murdered them or it could have been the assassin was preparing for the hit and was killed by the children. I just remember them as being a plot point that stuck out, and happened early in the book.
I believe it took place during wintertime. I think it had knights and mages and creatures the army fought in a forest.
The tone was serious, dark fantasy. The city had a medieval Europe feel to it.