I believe Montaro was made using the Construct 2 engine: https://www.scirra.com/

P.s.
If you're looking for an engine to use yourself, I would highly recommend Game Maker Studio 2: http://www.yoyogames.com/get (it's also available on Steam).

I love GMS2, as it has many similarities to UE4, but is far more accessible, without being dumbed down whatsoever.

The engine is completely free to try with the "Trial" version. However, there is NO time limit, giving you lifetime access. The only thing the the trial version limits you on is resource count, such a limiting you to 20 sprites, 15 sounds, etc., but these limitations are very generous and gives you plenty to work with.

If you have any questions, feel free to ask!