A stronghold belongs to whoever pays for it, and they are its master. If Cail the Wizard builds a tower, he gets to decide who gains its benefits. He may allow his sorcerer or warlock pals to do spell research there…until one or both betrays him! Or he, them! Only one character can gain the benefit of a given stronghold at a time, and each character can only benefit from one stronghold ability at a time.
If multiple characters chip in to pay for a stronghold—or one character just spends a lot of money—it is a castle and provides multiple benefits. Complex tasks take proportionally more time and money than simple ones, so a castle costs 10% more and takes 10% longer for every basic stronghold function it incorporates.
Jess, Anna, and Lars decide to pitch in together and buy a castle. It will contain a keep, a temple, and a tower. It will cost 33,800 gp and take 507 days. That’s 10,000 gp for the keep and 8,000 gp each for the tower and temple, 150 days for the keep and 120 days each for the temple and tower. Then an extra 30% on top (multiply time and cost each by 1.3) to account for the extra work necessary to incorporate many functions in one structure.
A castle cannot contain multiple of the same type of stronghold. If you build a castle with two towers, for instance, only one of them grants the benefits of spell research. Also, a stronghold’s benefit only applies to one character at a time. A character can switch between which stronghold benefits they receive by taking an extended rest.
The purpose of the design is, on the one hand, to somewhat model the real world where building a movie theater and automotive plant in the same building would (one assumes) take longer than the time to build each separately. For one thing the different kinds of strongholds have different requirements and therefore it takes more and different kinds of workers to complete them and the more workers you have the harder it is to coordinate them all.