I can confirm as a non-paid player, you can still go up very high on the leader boards and levelling up without paying for this freeium game (I have been in the top 200). I found the best strategy for non-paid players is max out all defences (towers and spells) before heading to the next throne room (it is very tempting to upgrade your throne room quickly).
In addition, to also time your upgrade cycles properly. For example, it takes me approximately 8 hours (two sessions of four hours = 1 collect) to max out my silo and “bread on hand”. I time my upgrades/free workers (I only have 3 workers) to coincide with the maximum amount of bread in my silo and “bread on hand”. I complete all my raids (use all my bread) within 5 minutes of the next free worker (there is only a 5 minute gap where other kings can steal my gold, however, I generally stay online for 5 minutes so there are no attacks). When my worker is free I upgrade a building or spell (I check upgrade times to ensure I can complete the cycle again, I leave 8 hour gaps for my three workers).
After my upgrades I generally have a zero gold balance. I don’t mind if other kings attack me after my upgrade because I have a zero balance in my gold chamber. Moreover I usually I earn about 20 gems per day, as other kings in my trophy range are required to scroll against my full base to get through.
Be patient and enjoy this fabulous game.