This arrow hail cannon can be used very well for storming castles.

First you have to let it charge, then, when you pressed "FIRE", the TNT ignites, the arrows fly down and are thrown away by the shock wave of the dynamite.

Instructions:

The construction is also described in such a video where I'm just too lazy to link. But the video is in an older version so trust me.so first you build like a TnT cannon a long water basin, at the edge of the basin are on-the-water-directed launcher on which is a redstone line (shift hold and place)the redstone line, comes from a button that is 4 blocks away from the launcher.

then you build a piston clock that activates a launcher with arrows...

now you build up 10 repeaters in a row (9x4Klicks,
1x1 click) its signal is reversed by a redstone flare. the redstone signal as a whole, runs from the button, to a plunger which is at the same height, opposite the thrower

then fill the launcher row with TnT and the other launcher with arrows. then turn on the Pistonclock until enough arrows are "drankleben" at the piston then turn it off again and FIRE! the piston contracts, the arrows fall down and exactly on the second explodes the TNT whose shock wave the arrows away.