4. Removing doors from vessels that are not attached to the ground and do not hold 40 Seah:[1]

If the hinges were only loosely placed in their sockets: The above discussion [in Halacha 3] was with regards to returning the door [back into its sockets], however to remove the doors from vessels is allowed in all scenarios that the hinge has not been strongly affixed into them.

If the hinges are strongly placed into their sockets: However if they are strongly affixed [into their sockets] then taking it apart involves [the prohibition of] destroying as will be explained.

The reason that this is allowed when placed loosely: This does not involve the [prohibition] of destroying [an item on Shabbos] because by vessels [as opposed to buildings attached to the ground] there is no [prohibition] against building and destroying.[2] However [the Sages] only said that [the Biblical prohibition of] building and destroying does not apply by vessels, if the vessel being built or destroyed is not sturdy. Such as for example removing the door    from [the socket of] a vessel and returning it [to its socket], that returning it is not [considered] a sturdy [form of] building being that [the door] is still [in truth] detached from the vessel as it was not strongly inserted and thus can be easily removed from it. Therefore when removing it one is not considered to be destroying a sturdy structure.[3]

Summary:

Is permitted to remove the door of a vessel which is not attached to the ground and does not hold 40 seah if the door is not strongly affixed to its socket. If it is strongly affixed to its socket then it is forbidden.

Q&A

May one remove the door of a toy house?

If it is strongly attached to its socket then it is forbidden. If loosely attached then it is allowed, so long as it is not attached to the ground and does not hold 40 seah.