The other guide is technically correct that you only need to buy 50 upgrades to your manor in order to unlock this achievement, but there are some tips and tricks to working to this goal.
-Upgrade costs increase across the board as you level up. So a Lvl 5 health upgrade while at Level 10 vs Level 20 will be very different. Therefore, you will need to be saving up more gold for each individual upgrade
-Due to the constant increase in cost, you'll want to consider ways to continue to build your income. As you get stronger, consider investing in the Miner class, Lich payment decrease, or increased coin upgrades, The Miner will give you a boost to each gold pickup, the Lich will stop taking all your money when re-entering the castle, and the boost will give more gold per pickup.
-If you beat the first boss (Giant Eye) before reaching Level 50, consider still going through the main castle area to collect more gold on subsequent runs. You should be able to get through it pretty easily and can collect a lot of extra gold. After that, work through the forest (area at the right side of the map) to collect gold.
With a little effort, you'll quickly collect the gold to make the necessary upgrades. The first 50 aren't overly expensive, but this should help speed it up a little bit as well.