So right now, a typical round of siege feels like a team deathmatch for the first 3/4 of the round. Then the defenders run out of spawns and gradually die, and the attackers either walk into the flag without opposition or run out of time.
I feel that there are three key mechanics from NW that made siege mode work that are absent in Holdfast atm.
1) 30 second spawn timer for defenders. Currently, defenders just spawn, charge, die, repeat. A long spawn timer would force them to fight more defensively, as they will quickly lose if they all charge forward and die the way they do now
2) Instead of a reinforcement pool for defenders, each player has a set # of spawns. This way, idiots who run out into the open can't waste all of their team mates tickets, and it will further encourage defensive play. Of course, as in NW, the server will have to be able to detect if somebody disconnects and reconnects to keep track of their spawns.
3) Overtime for a contested final point. This portion of mount and blade's sieges is always exciting, when the attackers make a final desperate lunge to seize the objective with the clock at 0:00.
Obviously making mechanic changes like this might require tinkering with either the round timer or spawn #s.