Though probably the easiest and least dynamic part of battle in M2TW, here is a strategy I used that really helps to minimize the casualties and increase your armies kill-per-minute ratio during the defense of a breach.
The idea is to make full use of missiles. Most breach defense tactics, illustrated above, limit the effectiveness of missiles due to lack of direct line of sight. So I wanted a tactic that allows direct line of sight between the enemy and my missile units/anti-infantry seige equipment.
Such a tactic is shown below. I don't know how else to describe it other than a 'funnel' tactic vs the above mentioned bowl/cup tactics. The funnel allows an opening at the end where missile soldiers have clear lane of fire. My preferred missile unit for maximum killing power is the Ribault. I find them brutally overpowering in such scenarios, and the the enemy never lasts long regardless of size.
* Weaknesses: Enemy can flank of course, coming over walls, forcing missile troops to task elsewhere and reducing effectiveness.
* If the enemy has a LOT of horse they may try to push forward. In that case I may sacrifice a unit of cheap spear to stand in the funnel to stop the horses and act as a sort of 'filter'. A level of friendly fire is tolerated as the ribault shreds both friend and enemy alike.
* The strategy is best employed on a bridge. The longer the funnel the better. I find only the first stage needs to be deep with infantry, the rest of the funnel can just be 3-4 man deep. This of course is reliant on how effective your missile units are. Anything with high damage and good rate of fire works. Catapults can even work but I'd suggest curl one edge of the funnel in slightly to screen it, as it can shoot over the screen just fine, while still allowing a slot for cavalry charges from one side for any enemies unfortunate enough to make it to the end of the funnel, and also allow missile troops to sneak in for a clear field of fire when the opportunity arises.
* For the more cautious or risk-averse, you can start the contact with a screen of infantry infront of some/all of your missile soldiers. Once the fighting has started and its clear the missile units are safe, reveal them by moving the screening infantry to the side.
* For larger battles, start the funnel quite narrow and allow swelling of the enemy position to naturally push your spears back and out of the line of fire.
* If you are using 2 units of Ribault against a large army, with this tactic they will net 300-400+ kills each. With good bow units instead perhaps half that number, in about double the length of time. The length of time is important as it means your (probably weaker) spears will be fighting the bulk of their army for longer, something to avoid.