LOL, YES, Buying Gems and BIG Buildings to Evolve L4s using the HMP will give you an “Imaginary” Advance! 
But, if you can not continue that kind of Investment, you will go back to where you belong! 
I have the ONLY HIGH Ranking City Populated with ALL L3s and CrownBuildings! 
I am a LEADER not a Follower! I can do my own MATH and Make My Own Decisions! I can Learn and Guide Myself! I see the TRUTH, not the Exaggrerations and Mistakes in the Information of Other Players! 
There are 2 Types of Buildings/Monuments!
#1 – A Building/Monument that is Under the Influence ot the HMP!
#2 – A Building/Monument that is NOT!
The HMP Reward for Buying a Building here is 4Days? And on a Monument Evolution is 2Days? I can find NO INFO Directly from CW concerning this Reward, but I see it Exist when I Collect Coins!
I consider My Buildings/Monuments that are PAST the HMP Days are “Normalized”! Basically, They are going to give me “Whatever” they are going to give me “FOREVER”, UNLESS I EVOLVE THEM? 
City Building is SO SIMPLE, 100 Gems at a time!
Use (2)T6 Building to Build L1(T7) Monument.
Use(9)of those L1(T7) Monuments to EVOLVE another L1(T7) Monument!
This will give you a L2(T10) Monument! Repeat (9) Above and Evolve this L2(T10) into a L3(T15) Monument! 
Your TB will Reward you and Soon you will be Buying T7Buildings(100Gems) to Speed your process! Then sets of (1)T9/(2)T8s for MORE Speed! 
Know THIS!!! TRUST the INFO CW Provides us! Do NOT TRUST Exaggerated/Emotional Info a PLR provides against CW Info!
I DO NOT CARE what any PLAYER here says about HOW “Magical and Powerful” their L4(T21) Monuments ARE! YES, During the HMP, CW Rewards them “Hansomely”! After THAT! (8) of my L3s give me the SAME as they get (If they Collect every 8Hrs). If they do not collect every 8Hrs, My L3s are MORE Productive?
I am NOT a GEM BUYER! I cant afford them! I would rather Eat and have Electricity! You CAN have MORE Success here if you BUY! But, Time will Tell ALL!