Jay Y.
Yelp
The Duomo is Milan's most beautiful church by exterior, but he has a competitor for the most beautiful church by interior: San Maurizio al Monastero Maggiore, a former nunnery!
Behind a virtually nonexistence facade, San Maurizio hides perhaps one of the most ornate church interiors and can actually compete with the famous Sistine Chapel. From the moment you step in the door - assuming you finally find the right door - you will be awed by the incredibly colorful paintings that surround you! Despite being in a small room, the vast amount of the beautiful paintings can spell-bind you for a while.
Additionally, for the first time in history, you as a public member can finally pass through the partition wall into the previously restricted nunnery hall! Like the public worship hall across the wall, the nunnery hall is also filled with paintings. However, while the worship hall paintings are intended to awe the public the divine grace with vibrant colors, the nunnery hall paintings intend to invoke piety with serenely-colored examples of female saints.
You can admire for a few minutes or examine for hours, but this church is certainly a sight that you should not miss! Did I miss it's free to enter?
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tl;dr version:
1) Perhaps Milan's most beautiful interior
2) Small spaces that are filled with marvelous paintings
3) Free sight that should not be missed