Right now you can bring invasions and megabeast attacks to a halt with a few well-placed cage or weapon traps, or a few dozen stonefall traps. There are a few things that can be done to make this a bit less gamebreakingly easy.
-Link weapon traps to power sources, or require them to be manually reset each time they're used. Also, limit the number of weapons that can be used in them.
-Make traps effective a certain percentage of the time, perhaps tweaked based on the attributes of the creature who triggered it. Cage traps should trap dragons 30% of the time or less, while perhaps they should trap goblins 75% of the time. This could be based on a number of the new attributes.
-Allow creatures in an invasion force to notice when one of their comrades has been affected by a trap and take appropriate AI actions (perhaps a chance to perceive traps inside a certain range, followed by avoiding spotted traps or maybe an increased chance not to be affected by them.)
-Let traps have a small chance of going off when triggered by a friendly creature - this would stop you from simply trapping your main entrance and calling it a day.
-Require most traps to be placed on indoor tiles. Right now a stonefall trap can be placed anywhere. This doesn't make much sense.
Any of these imho would make the trap situation a little bit more balanced.