The design is Hippodamian (look it up!) and laid out on a symmetrical grid of 500 x 700 metres. The plan is centred around 2 squares: la Place Royale, and la Place du Cardinal on which is the town hall and a covered market. It is surrounded by walls (made up out of the back walls of the outer ring of houses) and and a moat (now mainly grassed over). There are three monumental gates, with a fourth, ornamental gate, built to maintain symmetry. The rules for building in Richelieu were very strict - the purchasers of land had to commit to building there within 2 two years, and the house had to be built according to plans held at the clerk's office. Furthermore, anyone building had a choice between only 2 builders who were authorised to carry out building work. A register still exists which shows the list of the owners and all the works involved in building the town (and presumably which of the builders they chose).
We have been there twice, and it is still being resurfaced
The original market hall still exists, and is a huge oak and sycamore building which has been well maintained, but never really imporoved or renovated. This gives a real idea of the effort that went in to making Richelieu the most modern and comfortable of towns.
Outside the walls, the Cardinal built his palace, which was intended to be the largest in France. Most of the stone came from the Chateau at Saumur (which is why large parts of the ramparts of Saumur are being rebuilt, one assumes). The chateau no longer exists, but the gardens are still there, and you can hire bikes to ride them, or boats to explore the ornamental waterways.
A arial photo of Richelieu can be seen here.