First of all, the wording of this card suggests that it is an ongoing ability, so I'm going to treat it as such until someone corrects me.
Consider this scenario:
1) I play Siegeworks in battle
2) The numbers on my evil characters breach 20/*, so I discard an OT fort
3) Due to discard/return to hand/etc. an evil character leaves battle and my numbers drop below 20/*
4) I band in another evil character and my numbers breach 20/* again.
Do I get to discard another OT fort? I seem to have some vague memory that cards' abilities only activate once, but I'm not sure.
Siegeworks -
Type: Evil Enh. • Brigade: Multicolor • Ability: 3 / 0 • Class: None • Special Ability: If total abilities (*/*) of Evil Character(s) in battle reach 20/* discard one O.T. Fortress site. • Play As: When total abilities (*/*) of Evil Character(s) in battle reach 20/* during this battle, discard one O.T. Fortress.
