Just thought I'd give you guys some real life experience.
I do german longsword fencing and the mechanics are more of less the same as single sword and most other weapons of the time.

A strike over the top generaly beats a low strike unless the duellist striking low is both very quick and moves offline. The top strike has greater reach.
Moving offline (15 degrea's or more) gives a duellist strenght as long as his opponent stays online (straight).
A medium block will be fast and so would save you from high strikes.
If you mirror your opponent both swords will clash and you will wind (y-nd). where both swords turn round each other trying to gain the upper hand. this is where some nice dice rolling determines who wins.

This is a very basic run down but I hope it helps when working out the duels.