I know some of these ideas have probably been said before but having them said more than once will probably make them look more wanted 
1. Impassable windows made out of the current building materials available ( sandstone granite etc...) which have 3/4 health of a normal wall but can be shot over like a sandbag and provides cover for anyone shooting out of it.
2. trenches that act like a wall which the enemy has to bring ( or construct) ladders to cross during a raid.
3. a 2x2 tile structure which acts as a guard tower, giving resistances to anyone posted on them and giving them an accuracy bonus when using ranged weapons and allowing colonists to fire over walls which have no roof. It could also make them more vulnerable to fire and explosives due to the structure breaking?
4. a trade beacon which attracts more trade ships but also increases the chance of raids.
1. Impassable windows made out of the current building materials available ( sandstone granite etc...) which have 3/4 health of a normal wall but can be shot over like a sandbag and provides cover for anyone shooting out of it.
2. trenches that act like a wall which the enemy has to bring ( or construct) ladders to cross during a raid.
3. a 2x2 tile structure which acts as a guard tower, giving resistances to anyone posted on them and giving them an accuracy bonus when using ranged weapons and allowing colonists to fire over walls which have no roof. It could also make them more vulnerable to fire and explosives due to the structure breaking?
4. a trade beacon which attracts more trade ships but also increases the chance of raids.