There are certain things that become available with each level.. the higher lvl the spawn, the pricier the deco becomes, but the item that unlocks isnt always on the first level of the spawn... it could happen anytime a red candle pops.. just a matter of which one it will unlock on.
That is not my understanding of how it works. During the spawn,
one item of deco will spawn at
two of the different deco stages.
First stage - These items appear as soon as the spawn is started. They include brambles, the vats, levers, the iron maiden and the pedestal.
Second stage - These appear when the first red skull is added. They include the graves, the racks, the spikes, two runes and the small banana trees.
Third stage - Appears when the
second red candle of the
second level is added. Includes the lava/rock formation, the wall decoration, the sparkles and the soulforges.
Final stage - These appear when the
second red candle of the
third level is added. Includes the large banana tree, pillory, ore cart, executioner's axe and the remaining runes.
If you run through the spawn as normal, it seems that two items of deco are duplicated and can be taken away. However, when these items appear seems to vary with each spawn. So one spawn may produce a copy at the second and third stage, another may produce them at the first and fourth stage, etc. You can get extra items from the second to fourth stage by allowing the spawn to regress and then working it back up, but if an item did not spawn at a particular stage before then one will not spawn at that stage at any time during that spawn. For instance, if no item appeared at the second stage then it will not produce one there no matter how many times you work the spawn up and down.
It should also be noted that the item that spawns can be any object which is currently available, not just the items that appeared at that stage. So if an items spawns at, say, the third stage, it could be any items from the first, second or third stage objects. Consequently, getting the higher stage objects is relatively rare.
Here is a link to an Open Office spreadsheet file that has a list of the objects and when they appear. It is not totally complete - for instance there are wall torches that appear at some point but I'm not sure when or how many:
http://www.gilfane.com/Files/Lich_Spawn.ods