This is to encourage more defensive build and gives players more time when a player is being rushed.

Update 1.1
Houses have +25 HP per age (Starting from Tool Age)
Sentry Towers & Watch Towers have +25 HP (Starting from Tool Age)
Walls consecutively have +25, +50, +75 in bronze age (For each upgrade)
Town Centre's have +100 HP in Bronze Age
Wonders have +100 HP
Other buildings get +75 HP per age (Starting from Tool Age)

Patch Notes: Previously percentages stacked so you had houses with 500 HP in and Town Centres with 3000 HP Iron, this was way too much. Now TC's end up with 990 HP (With Architecture).

Version 1.0
All buildings HP are increased by 50% HP starting in Stone Age
Walls are increased by 25% HP in Bronze Age
Watch Towers & Sentry Towers are increased by 25%