An undeciphered letter is preserved in Papers connected with Lord Feilding's Embassy in Italy held by Warwickshire County Record Office as CR 2017/C157/4 (catalogue entry). According to the county record office's website, it is accompanied by the key with an explanation in French (CR 2017/C157/2), which allows revealing the plaintext of the letter in French.
The letter employs a polyalphabetic substitution cipher, which uses three substitution tables in turn. Each cipher permutes letters in pairs.
To encipher "soldat", the first letter "s", which is found to be paired with "r" in the first table, is enciphered as "r." The next letter "o" is sought in the second table and becomes "r"; the third letter "l" becomes "h" according to the third table. The first table was used again to encipher the fourth letter "d" to "i", and so on. The word "soldat" becomes "rrhiha" in the ciphertext. It can be seen that the same "r" in the ciphertext may correspond to plaintext "s" or "o" (or "b" if the third table is used), which makes codebreaking difficult.
To add further security, the first and third words in every line are nulls (i.e., to be ignored).
The ciphertext (as seen in an image at the county record office's website) is as follows (with nulls removed).
With the above key, this can be (more or less) deciphered as follows.
(Several errors are found in the ciphertext. Especially, the proper ordering in using the three tables is broken at least twice, as indicated with a check sign in the image to the right.)
Basil Feilding (Wikipedia) (styled Lord Feilding before succeeding his father as 2nd Earl of Denbigh) was King Charles I's ambassador to Venice, then Turin from 1634 to 1639 (Report p.vi, xi, xii).
Letters from Peter Morton, agent at Turin (Report p.vi), occasionally include words in cipher, of which a key is among the papers (Report p.12). It appears to be a figure code as was becoming common at the time (see another article). For example, "93" and "94" represent "Montagu" and "French ambassador", respectively (Report p.16).
Different kinds of ciphers may have been used with other correspondents (p.40 with Sir Alex Gordoun; p.44 with Duc de Candale).
His correspondence with foreigners was occasionally in French (Report p.13 with Monsieur Chateauroux; p.15 with Marquis de Pougny, French ambassador in England; p.24 with Syndicate and Council of Geneva; p.44 with Duc de Candale ). His secret correspondence with the Marquis Leganes and the Spanish governor of Vercelli (Report xii) may have been in French as well.
Civil War Ciphers: Lord Feilding's Mystery Coded Letter (Nearly) Cracked! (2016) (Warwickshire County Record Office)
CR 2017/C157/1-4 (catalogue entry) ... Location of the source material.
Report on the Manuscripts of the Earl of Denbigh: Preserved at Newnham Paddox, Warwickshire, (a.k.a. H.M.C.'s report on the Denbigh MSS or HMC's report on the Denbigh papers) Part V (1911) (Internet Archive)
CR 2017/C48 (catalogue entry) ... Despatches and letters received by Basil Lord Feilding as Ambassador in Italy, most of which are calendared in the Report above.