How do you build the small spiked walls?
Do you carry a box to a designated spot and build there or can they be built anywhere?

when you mean spiked walls do you mean like the iron ones?

if that's wha tyou mean only sappers can make them after they have unlocked the ability in the altar, after which you must get a certain amount of classpoints (the yellow orb in the bottom right corner) which you then spend to build the wall