The first weapon that could break the
Feudal charge, the Pike was mainly used by the
Flemish Mercenaries in the 15th century. It
revolutionised warfare, and spread quickly.
Formations of Pikemen were almost impossible to break unless you
flanked them, and were normally combined with a rear or flank rank of
archers to protect them from such. Pikemen normally went in about
five ranks, so the last pikeman's weapon only about a
foot from the arms of the front.
As it spread, many tricky and clever tactics were created to go with the long, strong shaft of wood and metal.
One of the most efficient tactics with the pike was not actually using them until the time was right. You would find a patch of long grass for your archers, get some scouts to drop a few pikes while the enemy slept, wait 'till the battle started and move an immobile unit (like archers) in. Of course, archers are prime targets for the opponent's cavalary and infantry- so they will be charged.
Now drop your bow and lift the pikes.
As horses can't slow well, and nor can angry, running men carrying very pointy things, you will probably manage to skewer your foe before they get a good slash in.
Other common tacics were ones like the use of it in charges- this tactic was hard to use properly, but with a good commander and precision timing, could reap units. Mostly used against other immobile units, the Pike would, unfortunately, have to be held further back to stop one accidently digging it into the earth and pole-vaulting. This would mean it would be difficult to use against other pikemen.
Charging pikemen front-on was general suicide. The only way to take them from the front was using units which could outrange the wall of points, such as crossbowmen and archers. Other tactics used were using heavily-armoured units that could just push away or ignore the "bristles" of the pikes; However, such units would fall prey to almost everything but pikemen.
Units such as Macemen, Greatswordsmen, and Axemen, using heavy, fatigueing, weapons could smash pikeheads, but even if they managed to break every single pike would normally get slashed apart by swift shortsword blows from the (no-longer)pikemen.