Does the value of LEGO increase over a period of time?
Generally speaking, yes. But there are many factors that determine how a given set will be priced over the years. Sets that have rare, more sought after pieces tend to cost more (think monorails), so if you'd happen to have a MISB monorail set you'd be rich. On the other hand, TLG can inadvertently mess with any investment plan by launching sets with similar themes or pieces, making your collection worth just as much as new sets.

Condition is also a key factor, MISB sets are on an entirely different level than opened, used, scratched, faded sets with missing pieces.

For more information i'd suggest looking at BrickPicker. There you can see that while the Diagon Alley has been slowly but steadily gaining value, the Town plan has basically stagnated. This could be caused by the value of a Harry Potter set for collectors, the unique pieces in that set, or any other factors.

There are also other questions in this topic here:


How can I find the value of a LEGO set over several years?
Historically, what types of sets have proven to be a great investment?
How can you estimate the value of a LEGO collection?