A Prison is a building in Tropico 3 and Tropico 4. Prisons supply cells to hold prisoners with. If there's no colonial forts around to turn into dungeons, prisons need to be built before the presidente can issue arrest warrants. Despite hiring policemen, prisons do not lower the level of crime in the area they're built in, since the policemen are tasked with being guards instead of being cops. However, the area around the prison will have slightly lower liberty, just like a police station. This can be reduced by approving the sensitivity training edict.