From reaching Level 2 of the expedition, we get Castle, which can be built on the water on our island.
Each castle level increases the population limit by 200 settlers.
The castle can be improved to the level 15.

Level 1	Level 2
200	400
Level 3	Level 4
600	800
Level 5	Level 6
1.000	1.200
Level 7	Level 8
1.400	1.600
Level 9	Level 10
1.800	2.000
Level 11	Level 12
2.200	2.400
Level 13	Level 14
2.600	2.800
Level 15
3.000