What the old sources meant by undoing
Traditional texts call this the house of self-undoing, which sounds like a curse and is closer to a description of mechanism. The twelfth is where you act against your own interests without noticing, because the behaviour is too familiar to register as a choice. Naming it is most of the work.
Solitude, willing and otherwise
The house covers places of withdrawal, from monasteries and hospitals to prisons, and also the ordinary version: the hours you need alone to function. A strong twelfth often describes someone who is sociable in company and needs considerably more solitude than they admit to.
Hidden opposition
Where the seventh house holds the enemy you can see, the twelfth holds the one you cannot: undermining, gossip, institutional friction, the thing working against you that has no face. It also holds your own contribution to that, which is the less comfortable half.
Planets in the twelfth
A planet here works, but out of sight, and is usually the last part of a chart the person recognises. Twelfth house Mars is anger that does not present as anger; twelfth house Venus loves privately and often at a distance; twelfth house Sun describes someone whose sense of self was formed somewhere nobody was looking.
These placements are commonly described as weak. They are not weak; they are unwitnessed, which is a different problem and a more tractable one.
Opposite the sixth
Sixth and twelfth face each other as the managed against the unmanageable. Everything that will not go on a checklist ends up here, which is why people with tightly organised lives so often find this house is where things eventually give.
Working with a twelfth house planet
The practical difficulty with a twelfth house placement is that you cannot observe it directly. The usual route in is other people's reports: the description of you that keeps recurring and that you keep disagreeing with is often a twelfth house planet being seen from outside.
The second route is circumstance. These placements tend to surface in solitude, in illness, in periods of enforced pause, which is why the traditional associations with hospitals and retreats persist. Not because those places are fated, but because they are the conditions under which an unwitnessed part of a chart finally gets witnessed.
Nothing here needs to be fixed. A twelfth house planet that has been named usually stops causing trouble and starts being useful.
Questions people ask about this
Is the twelfth house bad?
No. It is the least visible house, not the most damaging one. Planets there work out of view, which makes them harder to recognise rather than weaker.
Does it really cover prisons and hospitals?
Traditionally yes, as places of enforced withdrawal. The everyday version is simply the solitude you need in order to function.
Why do people find twelfth house placements hard to identify?
Because they run without conscious involvement. Other people usually see them long before the person does.