What is an Elven Prison? The entire city. Elves who trangress the Sacred Traditions of Elder Life (i.e. the Law in Human Speak), are bound to the city. They are given a slender band around their wrist that can not be removed by normal means. (This slender band can be other things, it varies from city to city). The band remains for the duration of the sentence. The band provides several compulsions.
1) Once banded, the person is unable to leave the city or enter the “quiet places” inside the city. This means they can not commune with nature to rejuvinate themselves as other elves do. They actually begin to age (slightly) because of this.
2) Related to the above, they feel the drag of time. Time is no longer a soft breeze that passes over them. They feel the grit and tear of time. This is the greatest punishment to an Elf. They now feel boredom and decay.
3) They are forced to reflect upon the errors of their way in their quiet moments. So if they are idle, they are forced to go over and over their crimes and punishments. This leads to 4..
4) They must also obey any command given to them by an Elf (or specific non elf residents of the city). Thus the prisoners are the “servant class” of the Elves. (Note: It is illegal to abuse the prisoners in most ways, so they are moderately politely treated). Servants actually like to be kept busy, so they will sometimes take on additional responsabilities gleefully to avoid or limit their exposure to 2 and 3. They are responsible for much of the building and scut work around the city.
PS: It is considered hugely impolite to mention one’s prisoner status, so people will participate in verbal gymnastics to skirt around the issues when it comes up.
Please note that it is very, very easy to violate Elven law. Those that have not broken it recently live like nobles. Those that have, are their serfs.