The gear Iron spikes description says:
Hammering one of these blunt, wide metal wedges between a door and its frame requires a standard action and adds 5 to the DC of checks to open the door. If nothing but a spike is holding a door shut, the base DC to open it is 15. Spiking a door is noisy; each time you drive in a spike, creatures within 20 squares can make a Perception check with a +5 bonus to hear you.
Let's assume the party tries to lock and prevent opening of door with DC 25 to unlock.
Is it safe to assume following interpretations?
Party uses one spike to increase the DC of the door.
Party uses two spikes to increase the DC of the door.
Bunch of guards tries to get to the party room and tries to bust open the door. The DC they need to beat is 30 for (1) and 35 for (2). They fail but remember they have the key and unlock the door and try to enter the room again. Now the DC is 15 for (1) and 20 for (2).
The main question is - does the DC increase of spikes stack? In my opinion RAW it should but I am not 100% sure about this interpretation. Given the amount of spikes one party can carry, they could easily create door with DC over 50 in one turn of spiking them.