Expansions are the plots of land in your Castle that are used to build your Buildings on. After the first two, they are unlocked in sets of 4 building spots, either in a 2x2 or 2x 1x2 layout and get progressively more expensive to unlock both in gold and time:
| || || |
|8 (final)||2,000,000||48 hours|
Unlocking time can be skipped at the cost of 1 gem per hour remaining. You can't unlock more than 1 Expansion in parallel. There seems to be no deeper reason to the different blue and red colored plots. They are just placed in a checker board pattern to indicate the limits. Every expansion will give you 4 spots. The 2 blue lined 2-spot plots in the middle form 1 expansion together. Placing Guard Towers strategically doesn't seem to matter since no one is rushing in the front door to attack. The Dark Prince will not counter attack.