1. Does Caissa chess implement ​"en passant" rule ?

Yes Caissa chess implements it, as well as any other chess rule known to ​FIDE.
Please check ​our "en passant" demo on youtube.

2. What is the strength level of Caissa's chess engine ?

We estimate the engine is capable of rendering decisions at 1900-2000 ​ELO level. This engine was designed to perform on mobile platform, with power efficiency in mind. We are working on ​CM mode, which will make Caissa significantly stronger.

3. Do I have to pay for subsequent update ?

No. If you purchased Caissa Chess, you are entitled for bug fix updates for the lifetime of the application. Meaning any 2.x updates will be free of charge. There is big feature 2.1 release coming. It will come free of charge.