Favorite medieval war tactic?

[Sir Erathor]

One of my favourite tactics was one used by Hannibal, the leader of the Carthaginian army, whilst at war with Rome (I know it's not Medieval, but it's still battle tactics). He burned Roman crops and houses, ravaging the whole countryside. This forced the Roman general (I can't remember his name), to take action. If he didn't do anything then he would be universally hated by the people (and possibly the Senate). Hannibal knew that he could win the battle, and the Roman army was, in fact, crushed. He used some other good tactics in the battle.
There's a whole lot more to the story than this, but this was just a rough write-up of what happened

[Sir Erathor]

Yes, sending burning oil down on he enemy from the battlements is a brutal, but very useful tactic.

A_V

That tactic was actually very rare. Murder holes in the gatehouse were employed, though. These were between the two potculli so that the enemy could be trapped and have missiles raining down on their heads as they tried to get in the castle.

Actually, it was almost always boiling water that was used, as oil could be extremely expensive.

[Aeridian]

I agree with Albatross, hit and run is good, and with a bit of sabotage cleverly mixed in it can do some considerable damage.

[Kev]

Heavy cavalry charge. Full armor, warhorses, etc. Would have made for an impressive sight.

[ForestFriend]

Hit and run type tactics are always good. Especially for lighter armed troops, archers etc. If you draw in heavy armoured fighters into unfavourable terrain, you can really hammer them. The Swiss farmer/soldiers achieved great early success against armoured knight and trained soldiers by superb use of terrain [charging downhill from forests into the flanks of the enemy]; using suitable weapons similiar to their work tools [axes] and also killing all who tried to kill, injure or enslave their peoples [by refusing to treat nobles as special and hold for ransom] . Very off putting for the privileged rich nobles. This led to early independence for the Swiss Confederation after their crushing defeat of Austrian Habsburgs in 1315 AD. An early and impressive use of people power .