As any fortress commander will tell you, the weakest part of a fortress is its gate. For this reason many races will build their gates from the strongest and most enduring materials available to them. This can include replacing wooden gates with iron ones, building several gates within the gateway, or constructing a portcullis. You can represent this with suitable modelling or simply tell your opponent before battle commences that the gate is reinforced.
If the fortress gate is reinforced, a -1 penalty on the Damage chart applies. So, to get a Crash! result and smash your way into the castle, you would need to get a total of 17+ on a reinforced gate instead of 16+.
Points cost: 20 points