I've tested and found a good way of fighting sally battles when defending (ie being the besieger)...

There are two variants of the same which I have named the Withdrawal Feint and the Plaza Rush...

In order to pull those through, you'll need to have at least a fast horse unit and preferably some missile units (or some more horse units) and preferably facing a garrison of infantry/missile units as lots of cav would disrupt the tactics (unless you have even more than the garrison).

The Withdrawal Feint :

1 - Drop all siege equipment.
2 - Withdraw all troops except the fast cav unit(s) and send them to the end of an imaginary battle line.
3 - Keep your other cav units a little behind your other troops
4 - When a little more than halfway to the map limit, stop and turn around your missile and inf units (by then the garrison should have completely come out and even started to pursue)
5 - As the garrison advances, it starts to form in lines of cav/inf/missile (sometimes sending cav to counter those cav you sent to the end of the line), use this to send you fast cav into the missile units disrupting and killing them (after having lost the enemy cav by doing a little loop some distance away)
6 - Have your missile units on "Fire at will" and wait for the enemy line to reach you, sending your cav to counter the garrison cav or to weak spots in the enemy
7 - Use your fast cav to chase any routers from the main enemy line and to charge in the back of units (rinse and repeat)
8 - The town should soon be yours.

The other tactic is a variant on the first one :

The Plaza Rush

1 to 3 - Exactly as before.
4 - Withdraw just enough to have the enemy advance on you and get away from the gates and
5 - Send the fast cav to the sides and if possible use some Mailed Knights also (A bodyguard could do the trick if a bit slow).
6 - Use the fast cav to kill missile units and use one or more to rush to the Plaza (hence the name ). Mailed knights are especially good for that part as they are quite fast and have some staying power, Vards might also do the trick but I've never plyed the Byz so I wouldn't know...
7 - Time it so that the enemy front line gets into firing range of your missile units.
8 - If timed just right, the enemy will turn around and try to get to the plaza, getting struck by arrows all the way to the gates.
9 - Send some heavy cav into their backs and use the unit(s) on the plaza to kill any routers trying to rally by reaching it
10 - Strike one for the besieger...

I've found that tactic to work mainly in the western theatre where I play mostly (French, Hungary, Danes) against other western factions.

Any and all observations are welcome...